Abstract <p>The article presents the results of studies on the phase behavior of <i>n</i>-nonadecane (C₁₉H₄₀) particles in an aqueous dispersion. The average radius of the <i>n</i>-alkane particles was 77 nm. The phase behavior studies were carried out using an original optical method. The experimental results (phase transition temperatures of melting, crystallization, and rotator phases) were compared with the results of studies performed using calorimetry methods. In the nanosized <i>n</i>-alkane particles, a rotator–rotator phase transition was observed, which was not observed in a bulk sample. This additional phase transition was also observed in other <i>n</i>-alkanes presented in the form of a nanosized aqueous dispersion.</p>