The properties of the adsorbed NHx species (x=0,1,2,3) on platinum(111) and (100)-(1x1) single-crystal planes are studied by the semi-empirical method of interacting bonds. Both surfaces reveal similar features. The adsorbed species NH and NH2 are stable on the surface, and stable NH3(ads) species cannot form. The NH2(ads) species is favourable in adsorbed hydrogen excess, but lack of the latter results in NHads becoming dominant. Both NH and NH2 species are expected to diffuse easily over the surface due to the small difference between their bond strengths to various adsorption sites.