Changes in seismicity parameters is often utilized as a tool for forecasting eruptive activity at volcanoes worldwide. One of these parameters is the slope of the Gutenberg-Richter law, known as the b-value, whose temporal variation is studied here as an indicator of volcanic activity. Four Alaskan volcanoes (Makushin, Martin, Redoubt, Spurr) were selected in order to reconstruct the b-value variation over a multiyear period (≥ 25 years). The magnitude of completeness and the b-value were estimated using a sliding window for each earthquake catalog, while bootstrap uncertainties were estimated for each window. The size of the analysis window was selected by applying the Magnitude Bandwidth Criterion (MBC) that maximizes the number of windows with magnitude bandwidth larger than 2.0 in the complete part of the catalog. All four volcanoes exhibit decreasing b-values before unrest and eruptions, while maximum b-values exceed 1.0 only at Redoubt and Spurr. Previously published laboratory experiments on volcanic rock deformation and failure suggest that the decrease in b-value is a result of increased stress that may stem either from magma intrusions or from exsolution of volatiles. On the other hand, whether the b-value at each volcano will exceed 1.0 or not, is also determined by the degree of material heterogeneity.