 Some preliminary experimental works have been carried out to investigate the nature of metachromasy of NSAIDs between bovine serum albumin (BSA) and phosphate buffer solution (PBS) at simulated physiological conditions (pH = 7.4 at 37 °C). Moreover, the linear dependence of absorbance and second derivative intensity of each free NSAIDs upon its concentration (Beer-Lambert's law) has been adequately emphasized to guarantee a reliable quantification of the binding constants (Ks) of NSAIDs to BSA.  Measuring absorption spectra of different concentrations of FLF, MCF, MEF and NIF in sample solutions containing various amounts of BSA. The results revealed that further quantitative spectral data for calculating the binding constants of drugs to BSA could not be obtained from these absorption spectra.  Upon calculating the second derivative, each NSAID shows bathochromic shifts with an increase in BSA concentration and derivative isosbestic points were clearly observed. The results indicate that the residual background signal effects are entirely eliminated in the second derivative spectra. Furthermore, the results clarified that each individual NSAID exists in two states which exhibit different derivative spectra, i.e., free drug in buffer phase and bound drug to BSA.  K values were calculated by fitting the observed data of the ∆D values and BSA concentration using a nonlinear least squares method with a Taylor expansion.