Eight compatibilizing agents were studied to investigate their effect on the quality of the interface between a phosphate glass fiber and a poly(lactic acid) (PLA) matrix. After application of the agents via dip-coating, the fibers were Soxhlet extracted to remove any unreacted compatibilizer. To assess the interface quality, single fiber tensile tests of treated fibers and interfacial shear strengths (IFSS) of single fiber composites (SFC) were assessed. Of the agents tested, Glycerol-2-phosphate disodium pentahydrate (GP) and low molecular weight PLA with a sodium salt terminal group (PLA-Na) showed the highest IFSS values, which were significantly higher than those of the control. Oligomeric PLA with a carboxylic acid end group and alendronate sodium trihydrate also showed an improvement over the control fibers. The hydrolytic degradation of these single fiber composites was studied over 7 days in water at 37 degrees C and a significant decrease in IFSS was observed in all cases, with the treated samples dropping to the level of the control. TGA and XPS analysis of the sized fibers showed that GP and PLA-Na had been applied successfully to the fiber surface. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3082-3094, 2010