In many applications, replacing a complex word form by its stem can reduce sparsity, revealing connections in the data that would not otherwise be apparent. In this paper, we focus on prefix verbs: verbs formed by adding a prefix to an existing verb stem. A prefix verb is considered compositional if it can be decomposed into a semantically equivalent expression involving its stem. We develop a classifier to predict compositionality via a range of lexical and distributional features, including novel features derived from web-scale Ngram data. Results on a new annotated corpus show that prefix verb compositionality can be predicted with high accuracy. Our system also performs well when trained and tested on conventional morphological segmentations of prefix verbs.