This study examines two nominalizing prefixes in Indonesian: PE-and PEN-, which derive nouns from verbs with a range of meanings similar to that found in-er suffix in English. The prefix PE-is form-invariant, whereas PEN-has several nasal allomorphs. Given their similarity in form and function, the question arises of whether PE-and PEN-are allomorphs. We conducted a corpus-based analysis of their productivity, using the written Indonesian corpus in the Leipzig Corpora Collection. In this corpus, PENis apparently more productive than PE-. Interestingly, the frequency of words with PEN-correlates significantly with the productivity of the corresponding base verbs. In addition, PENis more integrated into the verbal system; verbs that have PEN-are part of larger verb families. PEN-attaches almost exclusively to verbs and creates nouns denoting agents and instruments. By contrast, PE-creates nouns denoting agents and patients and attaches not only to verbs but also to nouns and adjectives. For derived words with PE-, there is no significant correlation between the frequency of the nominalization and the frequency of its base. PE-also does not participate in the linearity of the productivity of the allomorphs of base and derived words that characterizes PEN-. Words with PE-are also more often input to further reduplication and inflectional variants than is the case for PEN-. This corpus-based research thus illustrates that affixes can have different qualitative and quantitative properties, although at first blush they look like allomorphs. Our analyses justify their treatment in the Indonesian literature as separate prefixes.