Thermal desorption investigations on self-assembled monolayers (SAMs) had previously been carried out using techniques such as thermal desorption spectroscopy (TDS), scanning tunneling microscopy (STM) and X-ray photo-electron spectroscopy (XPS). In this paper, the thermal dissociation of alkanethiols (CnH(2n + 1)SH) at various chain lengths (n= 6, 12, 18) on sputtered gold layers was monitored in-situ using the Kretschmann surface plasmon resonance configuration on a spectroscopic ellipsometer. We found that the longest alkanethiol (C18) exhibits the greatest thermal stability, manifested by the least amount of angular shift, during heating, in the resonant spectral features. Predictions of desorption temperatures from SPRS for the longer chain thiols are in good agreement with XPS measurements.