1. METHOD FOR THE DETERMINATION OF THE TOTAL ACID NUMBER AND NAPHTHENIC ACID NUMBER OF PETROLEUM, PETROLEUM CUTS AND PETROLEUM EMULSIONS OF WATER-IN-OIL TYPE BY MID-INFRARED SPECTROSCOPY characterised in that it comprises the following stages: a) Submission of samples of petroleum, petroleum cuts and petroleum emulsions of water-in-oil type for analysis by infrared radiation at ambient temperature in a Fourier-transform infrared spectrophotometer operating in the mid-infrared band possessing an attenuated total reflectance accessory obtaining the respective infrared spectrums; b) Determination of total acidity of such samples through correlation between values obtained by means of potentiometric titration with potassium hydroxide and values obtained by means of the absorption spectrums obtained in foregoing stage a); utilising a multivariate regression model; c) Determination of naphthenic acidity of such samples through correlation between values obtained by means of solid-phase extraction and values obtained by means of the absorption spectrums obtained in foregoing stage a), utilising a multivariate regression model.