Phosphorescence spectra of phenanthrene and pyrene dopants in biphenyl are recorded at temperatures varied from 5 to 45 K. The spectra show an abrupt change at 17 K characteristic for the first-order phase transition. By further increase of the temperature, a continuous convergence of two Lorentzian spectral components is observed near 40 K. For the pyrene probe, the symmetrical splitting of the spectra exhibits a characteristic critical behavior, yielding a critical exponent of 0.41 and critical temperature 39.6 K for the incommensurate-commensurate phase transition. The phenanthrene probe does not precisely reflect the bulk structure of incommensurate biphenyl due to preferred distribution of phenanthrene guest molecules within domain boundaries.