NOVELTY - Determination of useful properties of individual elements (IE) of combinatorial library (CL), comprising a substrate and at least two IE in separated regions, by passing starting material(s) (SM) over CL to cause chemical and/or physical reaction of SM and analyzing effluent stream containing reaction product(s) and/or SM. Analysis step comprises obtaining and evaluating photo-acoustic signal(s).    USE - The process is especially used (claimed) for determining the activity, selectivity and/or storage stability of IE having catalytic activity. The reaction involved in the process (i.e. the potentially catalyzed reaction) is specifically decomposition of nitrogen oxides, ammonia synthesis or oxidation, oxidation of hydrogen sulfide to sulfur, oxidation of sulfur dioxide, direct synthesis of methyl chlorosilanes, oil raffination, oxidative coupling of methane, methanol synthesis, hydrogenation of carbon mono- or dioxide, conversion of methanol into hydrocarbons, catalytic reforming, catalytic cracking or hydrocracking, coal gasification or liquidization, heterogeneous photocatalysis, synthesis of ethers (especially MTBE or TAME), isomerization, alkylation, aromatization, dehydrogenation, hydrogenation, hydroformylation, selective or partial oxidation, amination, halogenation, nucleophilic aromatic substitution, addition or elimination reaction, dimerization, oligomerization and metathesis, polymerization, an enantio-selective reaction or a biocatalytic reaction (all claimed). Applications in the drug, food, nutritional supplement, feedstuff and cosmetic sectors are also possible.    ADVANTAGE - Electro-acoustic detection is simpler and more rapid than prior art detection methods (such as use ion-sensitive detectors), and is free of operating problems such as coking, polymer formation and decomposition of electrodes on long-term use. The process is readily automated.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for apparatus for the process, comprising the CL, source of SM and photoacoustic signal detection/evaluation device.    DESCRIPTION OF DRAWING(S) - The figure shows measuring apparatus for obtaining a photo-acoustic signal using a resonant or non-resonant measuring cuvette ('Messkuvette'), including a laser producing laser radiation ('Laserstrahl'), mirrors ('Spiegel'), gas inlets/outlets ('Gasein/auslasse') and a microphone ('Microfon'). (Drawing includes non-English language text).