The relaxation of the methylene blue monomer-dimer equilibrium in supercooled glycerol near the glass transition temperature, T-g, was studied by monitoring the change in the optical absorbance associated with dimer formation during continuous cool/heat cycles. A hysteresis is observed in the absorbance at approximate to T-g + 25 K. Based on an analysis of the data using the Tool-Narayanaswamy-Moynihan model, relaxation in the hysteresis region is exponential. Exponential relaxation and the occurrence of the hysteresis at temperatures above the calorimetric T-g are rationalized in terms of the length scale for monomer diffusion versus the length scale of the dynamic heterogeneity in glycerol. (C) 2003 Elsevier B.V. All rights reserved.