Glasses in the Li2O-P2O5 system with a molar ratio (R = Li2O/P2O5) from 1.0 to 2.4 are prepared by ultrafast quenching. Critical cooling rates are determined for the corresponding melts. The derived dependence of critical cooling rate on R differs in behavior from the liquidus curve of the studied system in the composition regions corresponding to the formation of individual compounds, because structural units of a melt undergo the reactions of interconversion. The equilibrium constant is first determined for the reaction of disproportionation of the terminal groups (Li2PO3.5) with the middle (LiPO3) and monophosphate (Li3PO4) groupings. A rather high value of the equilibrium constant for this reaction provides an explanation for both the incongruent melting of lithium diphosphate and relatively low critical cooling rate of the appropriate melt. Explorations into the nonisothermal crystallization of supercooled melts demonstrate that the dependence of the temperature T-max (T-max corresponds to a maximum of the crystallization peak for a melt) on the cooling rate V-cool can be represented as InVcool = A + B/(T-t - T-max)(2) where A and B are constants for the melt of a particular composition, and T-1 is the liquidus temperature. The data obtained by the computer simulation of nucleation and growth of crystals in a supercooled melt are in good agreement with the experimental results. The calculation and semiempirical methods are developed for determining the critical cooling rate.