This paper examines the irregular application of the sound change commonly known as 'Bantu Spirantization (BS)' a particular type of assibilation in front of certain common Bantu morphemes. This irregularity can to a large extent be explained as the result of the progressive morphologization (through 'dephonologization') and lexicalization to which the sound shift was exposed across Bantu. The interaction with another common Bantu sound change, i.e. the 7-to-5-vowel merger, created the conditions necessary for the morphologization of BS, while analogy played an important role in its blocking and retraction from certain morphological domains. Differing morpho-prosodic constraints are at the origin of the varying heteromorphemic conditioning of BS. These uneven morphologization patterns, especially before the agentive suffix -i, were entrenched in the lexicon thanks to the lexicalization of agent nouns. The typology of Agent Noun Spirantization (ANS) developed in this paper not only contributes to a better understanding of the historical processes underlying the varying patterns of BS morphologization and lexicalization, but also to internal Bantu classification. The different ANS types are geographically distributed in such a way that they allow to distinguish major Bantu subgroups. From a methodological point of view, this article thus shows how differential morphologization and lexicalization patterns can be used as tools for historical classification.