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The thermal stability of P-lactoglobulin was studied using differential scanning calorimetry (DSC) as part of a larger investigation into milk and whey fouling. Measurement of kinetics using DSC is very difficult as the resultant trace is the sum of simultaneous denaturation and aggregational enthalpies; the two processes were demonstrated by measurements at different heating rates. P-lactoglobulin denaturation proceeds via an intramolecular disulphide stabilized intermediate and is irreversible even before the denaturation temperature is reached. Denaturation and aggregational processes can be partially separated by measurements over a range of pH. The literature shows that the rate of fouling decreases as solution pH increases. However, DSC shows that the thermal stability of P-lactoglobulin decreases as pH increases, particularly above pH 6. It is hypothesised that aggregation rather than denaturation might be the critical step in the formation of fouling deposit.