The phase behaviour between -180 and +35°C at constant volume in CO 2CH 4N 2 inclusions includes some newly described phase transitions. All transitions can be [classified] ^ initial or final melting, homogenization, partial homogenization, sublimation or metastable homogenization. Phase sequences are classified in 9 groups, distinguished by the order and number of phase transitions. The variety of phase behaviour is explained by differences in fluid composition and molar volume. Phase assemblages are illustrated by isochoric (constant volume) phase diagrams of the binary systems CO 2CH 4 and CO 2N 2, covering the full range of compositions and molar volumes of interest for natural fluid inclusions. The topologies of V̄TX diagrams for the two binary systems are different and explain the unique phase behaviour of both systems, which can often be distinguished by their phase behaviour only. Both isochoric TX and isothermal V̄X sections are discussed. The diagrams allow the determination of molar volumes from microthermometric data and Raman analysis. Phase diagrams for the ternary system CO 2CH 4N 2 could only semi-quantitatively be established.