The colour characteristics (L*; a*; b*), dinitrosylhaem/total haem ratio (D), and pH of frankfurter- and Bologna-type sausages as influenced by 0%, 25%, 30%, and 35% of beef replacement with soy protein isolate were studied. The influence of soy protein hydration (1:4; 1:4.5; 1:5) on D was analysed for the sausages with a 15% replacement of beef. The colour characteristics of the sausages was influenced by the addition of soy protein isolates and particularly the lightness (L*) of the sausages was influenced by the amount of beef replacement with soy protein isolate. Soy protein addition to the sausage formulations changed the pH of sausage mixtures and altered haem conversion to dinitrosylhaem. The alterations of the conversion of haem to dinitrosylhaem were dependent on the degree of soy protein isolate hydration before incorporation to the sausage mixture.