Pre-mRNA splicing is regulated through the combinatorial activity of RNA motifs, including splice sites and splicing regulatory elements. Here we show that the activity of the G-run (polyguanine sequence) class of splicing enhancer elements is ∼4-fold higher when adjacent to intermediate strength 5′ splice sites (ss) than when adjacent to weak 5′ ss, and ∼1.3-fold higher relative to strong 5′ ss. We observed this dependence on 5′ ss strength in both splicing reporters and in global microarray and mRNA-Seq analyses of splicing changes following RNA interference against heterogeneous nuclear ribonucleoprotein (hnRNP) H, which cross-linked to G-runs adjacent to many regulated exons. An exon's responsiveness to changes in hnRNP H levels therefore depends in a complex way on G-run abundance and 5′ ss strength. This pattern of activity enables G-runs and hnRNP H to buffer the effects of 5′ ss mutations, augmenting both the frequency of 5′ ss polymorphism and the evolution of new splicing patterns. Certain other splicing factors may function similarly.