This study investigated the deposition and crystallization behaviors of electroless Ni-Cu-P deposits on aluminum substrates. The critical phosphorus content, 7 wt %, of transformation between crystalline and amorphous structure for Ni-Cu-P deposit has been identified. The concentration of copper salt (CuSO4) in the deposition solution greatly affects the Cu content of Ni-Cu-P deposit and thus the Ni and P contents. An excess CuSO4 concentration may impede the formation of amorphous Ni-Cu-P deposit. It was also found that the chemical composition and crystal structure of Ni-Cu-P deposits will vary during deposition. An amorphous Ni21.66Cu-11.60P deposit exhibits transformation at 366.5 and 461.4 degrees C when analyzed with differential scanning calorimeter. The Ni-Cu alloy and the Ni5P2 compound form preferentially from the amorphous Ni-Cu-P deposit during crystallization at 366.5 degrees C. The Ni5P2 phase further transforms to Ni3P phase upon interaction with Ni at 461.4 degrees C. The activation energies with respect to the two transformation reactions are 229.1 and 213.0 kJ/mol, respectively, for the crystallization of amorphous Ni-Cu-P deposit and the formation of Ni3P.