An orientational order parameter is proposed for the isotropic to smectic C-A(*) phase transition in antiferroelectric liquid crystals. A phenomenological theory is developed to describe the direct isotropic to smectic-C-A(*) phase transition on the basis of a free energy expansion. The free energy is written in terms of the coupled order parameters including the antipolar correlations. We present a detailed analysis of the different phases that can occur and analyze the question: under which conditions a direct isotropic to smectic-C-A(*) phase transition is possible when compared to other phase transitions? The theoretical results are compared with experimental results. (C) 2004 American Institute of Physics.