Visual observations of the co-existing phases over the temperature range from 311.40 K to 436.82 K and up to gas-liquid critical pressure for acetonitrile + butane mixtures have been made. Nine gas-liquid critical points, dew and bubble pressures, and densities at 20 K intervals from 333.15 K to 433.15 K and p-T-p-x relationships for three-phase liquid-liquid-vapor equilibrium were established. This system forms a minimum boiling azeotrope up to the gas-liquid critical point. On the p,T projection, the three-phase line terminates in a liquid-liquid upper critical end point, from which the liquid-liquid critical curve (of upper critical solution temperatures) starts vertically to high pressures.