We present a phenomenological approximation to the study of various non-universal features of liquid-liquid phase transitions in the framework of an approximate equation that relates the excess volume, the excess enthalpy, and the slope of the critical line. The mixtures under study are those containing 1-nitropropane (NP) or nitrobenzene (NB) and n-alkanes (CNH2N+2). The validity range of the above-mentioned equation is studied in detail for NP-CNH2N+2 systems. The consistency thus observed is within the experimental resolution limits, indicating that the equation can be extremely successful. The influence of effects at a molecular level on the slope of the critical line and on the critical amplitude of the isobaric thermal expansivity is analyzed from the microscopic interpretation of the excess volume and the excess enthalpy. This analysis is extended to NB-CNH2N+2 systems. The results reflect the importance of volumetric effects on critical behaviour, short-range anti-parallel ordering in NB appearing to be particularly relevant. (c) 2007 Elsevier B.V. All rights reserved.