The mechanism of electroless nickel deposition involves generation of hydrogen which can be entrapped in the NiP layer. In this study hydrogen evolution in several electroless composite coatings, that is, NiP-X (X = SiC, Al2O3 and boron particles), deposited on an aluminium (6063-T6) substrate, was investigated by the solid extraction method. It was found that particle codeposition can promote hydrogen occlusion in the layers, a fact correlated with the adsorption capacity and affinity of particles towards water or hydrogen itself. Hydrogen removal efficiency from coatings, after heat treatment, increased with the applied temperature (130, 160 and 190 degrees C for 1.5 h each). For the same heat treatment. 190 degrees C for 1.5 h), most composite coatings showed lower removal efficiencies (35-54%) compared to NiP layer (80%) and, as the amount of hydrogen in the composite coating increased, its removal efficiency decreased.