The formation of solid condensates by heterogeneous nucleation from a mixture of p-xylene and water vapor onto a cold substrate was observed by slowly cooling the substrate under reduced pressure conditions. The results from binary experiments were compared to those for the pure vapors. Two critical substrate temperatures, corresponding to the onset of nucleation, were determined experimentally at a given initial vapor pressure by observing a rapid decrease in the vapor pressure. A mixed vapor with a water content greater than 85% formed ice at the first critical point, and the critical partial vapor pressure agreed well with that for pure water. For mixed vapors in which the p-xylene content was greater than 30%, p-xylene crystals formed at the first critical point, but the critical vapor pressure tended to decrease with decreasing p-xylene content. For mixed vapors with a composition close to the eutectic, it was unclear which crystal formed at the first critical point. The relationship between the logarithm of the critical vapor pressure and the inverse of the critical temperature at the second critical point showed good linearity and was close to that at the eutectic condition. Using these experimental results, we discuss the conditions required for separating binary eutectic-forming vapors by sublimation.