The smectite to illite transformation in early Miocene sediments was studied in wells and outcrops in the western Pannonian Basin, where magmatic activity caused very high Miocene heat flows (250-400 mW/m2). Although the thermal history is similar, large differences in smectite to illite diagenesis were observed. (1) The boundaries between early/middle and middle/late diagenesis in two wells (Pichla, Radkersburg) and the Maribor area correspond to vitrinite reflectance values of 0.4-0.8 and 1.1-1.5% Rr. Anchimetamorphism starts at ~2.1% Rr. In spite of magmatic heating, the smectite to illite transformation can be modelled using kinetic data proposed for areas with a 'normal' burial diagenesis. (2) Smectite to illite reactions are advanced compared to vitrinite reflectance in the Ribnica-Selnica Trough. This is probably related to fluids with elevated K+ concentrations. (3) Clay mineral alterations lag behind coalification significantly in the Mitterlabilt well. These different correlations indicate that clay mineral thermometry should be used with caution and that factors other than temperature and time can influence clay mineral reactions.