Variations in the critical micelle concentration (CMC) have been determined for sodium dodecyl sulfate and sodium deoxycholate (NaDC) in the presence of five drugs and polyethylene glycol (PEG) at temperatures (298.2, 304.2 and 310.2) K. From these data, thermodynamic parameters associated with the micellisation process (DmicG, DmicH and DmicS) were calculated. In the presence of some drug-based compounds, the CMC of SDS was affected, for example the presence of PEG dramatically reduced the CMC in all cases. Furthermore, PEG appeared to reduce the enthalpy of micellisation for all scenarios with only comparatively minor variations in the change in Gibbs free energy for the processes observed. For NaDC, the calorimetric results were far less predictable. A primary aggregation event recorded at a comparatively low concentration failed to appear for NaDC in the presence of a secondary compound, such as a drug or PEG. For NaDC, the presence of PEG had little effect on the CMC and corresponding thermodynamic data.