NOVELTY - A gas diffusion substrate comprises a porous structure and a filler material. The filler material comprises: (i) a base filler material which is electrically conducting and carbonaceous; and (ii) one or more modifier materials which are hydrophilic in nature relative to the base filler material.    USE - The porous gas diffusion substrate and porous gas diffusion electrode are used in electrochemical devices e.g., fuel cells (claimed), metal-air batteries and electrochemical gas sensors.    ADVANTAGE - The porous substrate has good gas diffusion properties and improved water management properties.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (a) a gas diffusion electrode comprising a gas diffusion substrate as above and an electrocatalyst material:    (b) a membrane electrode assembly comprising a gas diffusion electrode as above, a second gas diffusion electrode which may or may not be an electrode as above, and a solid polymer membrane;    (c) a membrane electrode assembly comprising a gas diffusion electrode as above, a gas diffusion substrate which may or may not be a substrate as above, and a solid polymer membrane, in which an electrocatalyst layer is applied to the side of the membrane facing the gas diffusion substrate;    (d) a membrane electrode assembly comprising a gas diffusion substrate as above, a gas diffusion electrode which may or may not be an electrode as above, and a solid polymer membrane, in which an electrocatalyst layer is applied to the side of the membrane facing the gas diffusion substrate;    (e) a membrane electrode assembly comprising a gas diffusion substrate as above, a second gas diffusion substrate which may or may not be a substrate as above, and a solid polymer membrane, in which an electrocatalyst layer is applied to both sides of the solid polymer membrane;    (f) a fuel cell comprising a gas diffusion substrate as above; and    (g) a fuel cell comprising a gas diffusion electrode as above.