A thermodynamically radical scavenging activity (RSA) of vitamin B niacin (NCN), niacin amide (NMD) and vitamin C ascorbic acid (AA) using 1,1-diphenyl-2-picrylhydrazyl (DPPH) through spectrophotometric and physicochemical methods at T = (298.15, 303.15 and 308.15) K has been studied. New experimental data on density, sound velocity, isentropic and apparent molal compressibility of vitamin + DPPH solutions are reported as a function of temperature and concentration. The results are discussed with regards to their structure-activity relationship (SAR) principles and relative deviations in RSAs with the structural potentials of the vitamins have been analyzed by comparing both the spectrophotometric and physicochemical results.