There is no conclusive model for the emission mechanism for Gamma-Ray Bursts (GRBs). The conventional synchrotron emission models have been shown to present difficulties in explaining the hard low-energy spectral slopes (e.g., Preece et al. 1998) and the sharp peak curvature (e.g., Yu et al. 2015) in the observed time-resolved spectra. On the contrary, thermal emission from the photosphere (whether or not re-processed by sub-photospheric dissipation, which results in a broadened Planck function) provides a natural explanation for most of the narrow time-resolved spectra (e.g., Ryde 2004; Pe'er & Ryde 2011). The peak energy and low-energy spectral slope of some GRB spectra are shown to exhibit correlation (Burgess, Ryde, & Yu 2015). We investigate such behaviour by performing time-resolved spectral analysis using Bayesian techniques (Vianello et al. 2015) and compare the results to various emission models including the thermal emission models.