Selected electrodeposition conditions were used to prepare crystalline and amorphous Ni-P coatings with a homogeneous phoshorus content across the sample. Aim of the research is to find the percentage of phosphorus at which the transition from a crystalline to an amorphous as-plated structure takes place. X-ray diffraction analysis showed that the structure is x-ray amorphous greater-than-or-equal-to 11.6 weight percent (w/o) phosphorus. At 11.6 w/o phosphorus, however, the electron diffraction pattern gives clear indication of a crystalline structure. At higher weight fractions of phosphorus only diffuse electron diffraction rings characteristic of an amorphous material were detected. Dark field electron microscopy still revealed the presence of small isolated crystallites in these amorphous Ni-P coatings. In a TEM with a hot-stage facility, two different-transformation mechanisms were observed, namely, nucleation and dendritic growth greater-than-or-equal-to 13.1 w/o phosphorus, and grain growth up to 11.6 w/o phosphorus. In this way, it is established unambiguously that for electrodeposited Ni-P coatings the transition from a crystalline to an amorphous as-plated structure takes place between 11.6 and 13.1 w/o phosphorus.