NOVELTY - A method for limiting the maximum strain developed in a mixed ionic conductive ceramic membrane (I), subjected to chemical and/or thermal strain during a transition phase, involves (a) varying the rate of modification of (i) the oxygen partial pressure from the oxidizing and/or reducing side of (I) and (ii) the total pressure gradient between the oxidizing and reducing sides and/or (b) the rate of modification of the thermal gradient at the surface of (I).    USE - The membranes (I) are used in catalytic membrane reactors (CMR's), specifically for separation of oxygen and conversion of hydrocarbons into more valuable products (especially conversion of methane into synthesis gas). In particular oxygen is separated from air by diffusion in ionic form through (I) and reacted directly with natural gas (mainly methane) on catalytic sites (particles of nickel or noble metal) deposited on the surface of (I) to give synthesis gas having a hydrogen to carbon monoxide molar ratio of 2:1, i.e. the ratio required for conversion of the synthesis gas into liquid fuel by a GTL (gas to liquid) process.    ADVANTAGE - The strains on the (fragile) membranes (I) are minimized during the phases of greatest strain, i.e. phases of transition between stable states, especially during reactor start-up and shut-down phases. Chemical and thermal dilatation of (I), and consequent reliability and safety problems, risks of membrane damage and reduction in the membrane operating life, are thus minimized. The strain limiting procedures do not themselves involve risks of membrane damage, unlike prior art material flow rate-based methods.    DETAILED DESCRIPTION - A method for limiting the maximum strain developed in a mixed ionic conductive ceramic membrane (I), subjected to chemical and/or thermal strain on its oxidizing and/or reducing face during a transition phase, involves (a) controlling the chemical strain on the face(s) by varying the rate of modification of (i) the oxygen partial pressure from the oxidizing and/or reducing side of (I) and (ii) the total pressure gradient between the oxidizing and reducing sides and/or (b) controlling the thermal strain by varying the rate of modification of the thermal gradient at the surface of (I). INDEPENDENT CLAIMS are included for:    (1) a method for starting up a reactor containing (I), involving (A) determining the Young's modulus, breaking strain and strength of (I), (B) determining the maximum strain as a function of the rate of modification of the oxygen partial pressure from the oxidizing and/or reducing side, the total pressure gradient between the oxidizing and reducing sides and the thermal gradient at the surface, (C) comparing the maximum strain with the measured breaking strain and (D) introducing first and second gas mixtures in such a way that the maximum strain is always kept below the breaking strain, using the above maximum strain limitation method;    (2) a method for shutting down a reactor containing (I), involving stages (A) - (C) as above, followed by (D') introducing a third gas mixture from the oxidizing and/or reducing side (optionally followed by introducing oxygen from the reducing side) until the atmosphere is identical on both sides of (I), in such a way that the maximum strain is always kept below the breaking strain, using the above maximum strain limitation method; and    (3) a method for producing synthesis gas containing hydrogen and carbon monoxide, by pre-reforming a hydrocarbon mixture and reforming the obtained hydrocarbon mixture using in a ceramic membrane catalytic reactor (CMR), the novel feature being that the reactor is started and/or shut down using the above process(es).