The blending of established biodegradable polymers offers the potential to fabricate new polymeric materials whose erosion kinetics can be modified by varying the composition of the blend. However, the immiscibility of most polymer blends complicates the erosion kinetics because of the influence of phase morphology on degradation. In this paper, we describe how the simultaneous acquisition of atomic force microscopy (AFM) and surface plasmon resonance (SPR) data can be used to interpret the effect of phase morphology on erosion. We have analyzed the degradation of thin films of blends of poly(sebacic anhydride) (PSA) and poly(DL-lactic acid) (PLA) using a combined AFM/SPR instrument, which enables the dynamic changes in surface morphology resulting from polymer degradation to be related to the SPR recorded kinetics of film erosion. This analysis has demonstrated three stages in the erosion of the films at pH 11, with the rapid loss of PSA dominating the initial stage of erosion, the slow loss of PLA detected in the final stage, and an intermediate stage displaying an extended transitional period during which the rate of erosion has an intermediate value. The effect of blend composition on phase morphology and hence the relative importance of these three stages during erosion are explored.