The a.c. electrical and catalytic properties of YBa2Cu3O7-x have been investigated by in situ dynamic experiments. The data indicate a strong dependence of surface conduction on chemisorbed spec ies which, if loosely bound, cause fluctuations. Proton-donating compounds produce an increase in conductivity and an oscillating behaviour. Fluctuations can also be associated with instability of the basic matrix at low temperatures in various atmospheres, and may explain the somewhat erratic catalytic properties of this material. In particular, the presence of water may lead to blocking of some adsorption-active catalytic sites.