The reduced-mass dependent nature of H35Cl and D35Cl were compared via FTIR analysis by analyzing and ultimately comparing the occurring spectral lines measured in wavenumber. Comparison of these data showed the distinct changes that occur when the reduced mass of a compound is changed, expressed as molecular constants. The calculated ratio of band origin between the HCl and DCl were also compared to the theoretical value. In most cases, the constants agreed with the literature values. The differences found between the calculated ratio of 1.3803 versus the theoretical ratio of 1.3950 were found to be insignificant, with only a 1.06% difference. Finally, a simulation of H35Cl and D35Cl with inclusion of known molecular constants confirmed the successful outcome of this experiment where wavenumbers predicted were nearly the same as the experimental values. There were of course some anomalies in the analysis done with the spectral data. There was poor linearity of the DCl plot of [uR(J) - uP(J+1)] / (J+1) in respect to (J+1)2, where R2 = 0.55. As a result of the poor linearity and the dependence of the calculation of centrifugal stretching constant D on the slope of the best-fit line, DDCl = 2.47244. This was a large deviation from the known literature value.