In this work, a revision of a predictive model previously proposed by Coniglio for the calculation of pure component vapor pressures by means of the Peng-Robinson equation of state is presented. A new expression for the shape factor m, more suitable for heavy components, has been suggested by Trassy and a simplification of the group contributions is proposed. In this part I, the method is applied to alkanes, naphthenes, aromatic hydrocarbons, alkenes and alkynes. Results obtained on vapor pressures are compared, when possible, to those obtained using the previous versions. A prediction for heavier hydrocarbons is also presented. This new version offers a good compromise between good results and simplicity. (C) 2003 Elsevier B.V. All rights reserved.