Liquid-liquid miscibility temperatures, as a function of composition, have been determined experimentally for the binary systems formed by ethanenitrile (acetonitrile) with octane, nonane, decane, undecane and dodecane and butanenitrile with octane, decane, dodecane, tetradecane and pentadecane. This study was also extended to include binary systems of pentanenitrile with long-chain alkanes, however, no liquid-liquid phase separation was observed from room temperature down to 270 K. All the measured systems present solubility curves characterized by asymmetry with respect to equimolar composition and the presence of an upper critical solution temperature (UCST). The experimental results show that for a given set of binary mixtures with a common nitrile the solubility diminishes with increasing alkane chain length, which is a clear manifestation of increasing non-ideality, and for mixtures with a common alkane the solubility increases with increasing nitrile chain length, which in turn is evidence of the decreasing effective polarity of the nitriles as their chain length increases. The Weimer-Prausnitz modification for polar components of Hildebrand’s Regular Solution Theory incorporating a Flory-Huggins entropy of mixing has been used to calculate the UCST for the ten systems measured and these values compare very well with those obtained experimentally considering that no adjustable parameter is included in the theory. The theory was also used to calculate the critical composition and qualitative agreement is observed with experimental data.