The N-2 and H-2 evolution, respectively, were monitored during deposition of Pd and Cu from electroless plating baths to obtain in-process control of the composition during preparation of 3-7 mu m thick PdCu membranes on tubular ceramic substrates. Compositions estimated by gas evolution compare favorably to those measured in post-mortem XRD and EDS analyses, mostly differing by not more than 1 at.%. This result suggests that use of gas evolution measurements to enable in-process control of composition to within 1 at.% is feasible. Annealing experiments in an H-2 atmosphere demonstrated that, at 893 K. only 48 h are needed to form a stoichiometrically homogeneous, 9.5 mu m thick, face centered cubic (fcc) Pd63Cu37 membrane from sequentially deposited layers; at 723 K, the same transformation requires over 2 weeks. The appearance of transient body centered cubic (bcc) and fcc phases with lower Pd contents signaled compositional segregation in the initial stages of alloy formation at 723 and 773 K and could be a source of persistent stoichiometric heterogeneity particularly in bcc PdCu membranes. The H-2 fluxes of fcc Pd58Cu42 and Pd70Cu30 membranes were J(H2) = (1.6 +/- 1.1) mol m(-2) s(-1) exp[(-24.8 +/- 0.4)kJ mol(-1) /RT] and J(H2) = (3.7 +/- 0.6) mol m(-2) s(-1) exp[(-21.3 +/-1.0)kJ mol(-1)/RT], respectively, at 100 kPa H-2 pressure difference. (c) 2008 Elsevier B.V. All rights reserved.