The relationship between three parameters, the Tmax given by Rock-Eval pyrolysis, the illite content in illite-smectite mixed layers (I-S) and the Kübler Index (KI) has been investigated in the Cretaceous–Neogene, sedimentary syn-orogenic successions in the Northern Apennines (Italy). A strong relationship was found between maturity stages of kerogen, illite content in I-S and KI. The oil formation zone for continental organic matter (Type III), delimited by Tmax between 434 and 465°C, corresponded to rocks with short-range ordering R1, I-S with illite content between 60 and 85% and KI values in the range 0.85–0.65 (°Δ2θ). Over-mature rocks were characterized by Tmax >465°C, a long-range ordered I-S with an illite content >85% and KI in the range 0.65–0.45 (°Δ2θ). The relationship permits use of both mineralogical parameters and Tmax to estimate palaeotemperatures in sedimentary successions and it can be exploited in hydrocarbon research to evaluate the petroleum potential.