The thermoanalytical purities of two sulfapyridine samples, drug (I) and standard (II), have been studied with different methods. By transparency measurement the fusion temperatures and intervals indicate a lower percentage of impurity for (II) than for (I). By means of two thermal analyzers the authors have shown the good agreement of the results: 99.790__ 0.047~ and 99.800+ 0.045700 (I). Heat-flux differential scanning calorimetry is the only method able to give precisely the degree of purity of (H): 99.956 + 0.028 ~, this value being a limit for the power-compensation differential scanning calorimetry analysis (DSC 1B).Dans le cadre de l'6tude thermoanalytique de m6dicaments organiques [1-3], le taux de puret6 de deux 6chantillons de sulfapyridine a 6t6 d6termin6 par analyse thermom6trique.La mesure de la transparence, l'analyse calorim6trique diff6rentielle ~t compensation de puissance, l'analyse calorim6trique diff6rentielle ont 6t6 raises en oeuvre.Leur principe repose sur l'abaissement de la temp6rature de fusion en pr6sence d'impuret6, donn6 par la relation de Van D'une fagon g6n~rale une amelioration des r6sultats est obtenue en augmentant le nombre des fractions de substance fondue 6valu& et en recueillant le maximum de donn6es.