Phase equilibria phenomena such as cosolvency effects, miscibility windows, and non-critical (liquid + gas) (l.g.) holes in the critical surface in (carbon dioxide + a 1-alkanol + an alkane) were studied up to 100 MPa. In continuation of experiments by Kordikowski and Schneider and Pohler et al., carbon dioxide systems containing a 1-alkanol withm⩽ 10 and an alkane with n⩽ 16 (where m and n denote the number of carbon atoms in the alkyl chains of the 1-alkanol, or the alkane, respectively) were investigated for large cosolvency effects. An isothermal cosolvency effect of 14 MPa has been found in (carbondioxide + 1- octanol + hexadecane) at T = 298 K with additional exhibition of large isobaric miscibility windows. Non-critical l.g. holes in the critical surface are also experimentally established. These l.g. holes in the critical surface, which have already been described as l.g. two-phase holes in the (liquid + liquid + gas) (l.l.g.) three-phase surface by Patton et al. and Peters et al., are now deduced from the occurrence of cosolvency effects when cosolvency lowers the critical surface of a ternary system onto the l.l.g. three-phase surface. This can ultimately lead to a penetration of the l.l.g. three-phase surface by the critical surface with the exhibition of a, mostly closed-loop, critical end point locus as the intersection line, surrounding a non-critical l.g. region. The relation between miscibility windows and holes in the critical surface is illustrated by the examination of systems exhibiting both miscibility windows and holes such as (carbon dioxide + 1-heptanol + pentadecane) and (carbon dioxide + 1-decanol + tetradecane).

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