<p>Changes in the phosphorescence intensity of acenaphthene in supercooled toluene during heating at a rate of 0.04 K/s at concentrations of 2∙10^–3 and 2∙10^–2 M are studied. It is shown that when the glass-transition temperature of toluene <i>T</i>_<i>g</i> = 117 K is reached, effective quenching of phosphorescence by oxygen begins for both solution concentrations. The intensity increases at <i>T</i> = 130 K because of the appearance of a crystalline phase. The values of these temperatures are the same for both concentrations. An “explosive” intensity increase is observed at <i>T</i> = 140 K for the solution of concentration 2∙10^–3 M because of self-acceleration of toluene crystallization. Such changes are not observed for the solution of concentration 2∙10^–2 M. Possible causes of the established changes in the phosphorescence intensity and the use of this effect to study structural changes in supercooled organic solvents are discussed.</p>