The mechanism responsible for protein haze formation in wines remains essentially to be elucidated. Current knowledge suggests the absolute requirement of one or more as yet unknown non-proteinaceous wine components (termed the X factor) for protein precipitation in wines. Using the single grape variety Arinto wine, naturally containing 280 mg protein/l, a series of heat stability tests were performed over a range of wine-relevant pH values (from 2.8 to 3.8). The results obtained indicate the existence of at least two different mechanisms responsible for the heat-induced precipitation of the Arinto wine proteins: one occurring only at the higher pH values, that appears to result from isoelectric precipitation of the proteins; another prevailing at the lower pH values, but possibly operating also at other pH values, that depends on the presence of the X factor. Therefore, conclusive evidence is provided for the existence of the X factor, here defined as one or more low molecular mass wine components that sensitise proteins for heat-induced denaturation at low wine pH values and whose presence is a pre-requisite for the precipitation of proteins in wines under these circumstances. The chemical nature of protein aggregation was further analysed as a function of pH. Neither of the two proposed mechanisms responsible for the heat-induced precipitation of the wine proteins is electrostatic in nature, lectin-mediated or divalent cation-dependent. Both mechanisms show minimum turbidity at pH 7, but increased turbidity towards lower and higher pH values.