A lumping and delumping method is useful for estimating the properties or behavior of liquid and/or vapor hydrocarbon phases from data relative to a reference set of hydrocarbon mixtures in a series of thermodynamic states in a medium. Each one of the hydrocarbon mixtures is grouped into at least three constituents (V, I, H), considering that the gas phases resulting from separation under conditions referred to as surface conditions of each mixture do not contain (H) and that the oil phases do not contain (V). The compositions of the separation products are determined by material balance. The at least three-constituent composition of each hydrocarbon mixture is determined by combination of the products resulting from the separation in proportion to the amounts of each separation product. Delumping is performed to predict a detailed composition of a fluid in the medium.