The Ni–P alloys are widely used as catalysts, magnetic and wear resistant materials. Properties of the nickel-phosphorous alloys are defined by the composition. A highly regulated technique to obtain the alloys with specified composition is the electrosynthesis. It is a relevant task to establish dependences of the alloy composition on the process conditions. In the present work we examined the influence of electrolysis parameters on the Ni–P alloy composition, obtained from the methanesulfonate and sulfate electrolytes. It is shown that an increase in the concentration of sodium hypophosphite, acidity and temperature of the electrolyte increases phosphorus content in the alloy. It was established that when carrying out the electrosynthesis under galvanostatic mode, a change in the alloy composition is predetermined by the rate of phosphorus formation. Atomic phosphorus is formed as a result of the course of two reactions. There occurs the electroreduction and disproportionation of hypophosphite-anion involving hydrogen ions. An increase in the concentration of hydrogen ions in the near-electrode layer contributes to an increase in the rate of phosphorus formation and growing phosphorus content in the alloy. That is why the alloys with a higher content of phosphorus are formed at lower pH indices of the electrolyte and at higher temperature. It was established that weak buffer properties of the methanesulfonate electrolyte are responsible for the lowered phosphorus content in the synthesized alloy. High pH index in the near-electrode layer reduces the rate of phosphorus formation. Established regularities might prove very useful when designing new technologies of the Ni–P alloy electrosynthesis of specified composition from the methanesulfonate electrolyte.