The objective of this investigation was to determine the effect of pH and sodium chloride added on the thermal denaturation of oxy- and methaemoglobin. An oxyhaemoglobin, obtained from cattle blood, was changed into a methaemoglobin under a process of oxidizing this pigment using a potassium ferricyanide. A denaturation temperature of the two forms of haem pigment was determined in water solutions showing a pH value ranging from 5.0 to 7.0, as well as in solutions of pH = 5.6 and pH = 6.8 with added sodium chloride amounts ranging from 0 to 6%. For the purpose of determining the denaturation temperature absorption spectra of heated pigment solutions were analyzed. When absorption peaks, defined as characteristic for a given pigment form, disappeared, it was an indicator of the pigment’s thermal denaturation. A solution’s pH value had a significant impact on the denaturation temperature of both the oxy- and methaemoglobin. The denaturation temperature of the two pigment forms rose along with the growing pH value of the solution. However, in the case of oxyhaemoglobin, its denaturation temperature depended more significantly on the solution’s pH if compared with the methaemoglobin. As for solutions showing pH equaling from 5.4 to 5.8, the two pigments were denatured at a temperature of 64 to 66°C. When NaCl was added to the solution of pigments, their denaturation temperature decreased; the decrease in the denaturation temperature in the solutions of pH = 5.6 was higher than in solutions of pH = 6.8.