Sometimes, the relationship between form and meaning in language is not one-to-one. Here, we used event-related brain potentials (ERPs) to illuminate the neural correlates of such flexible syntax-semantics mappings during sentence comprehension by examining split-intransitivity. While some ("rigid") verbs consistently select one auxiliary, other ("flexible") verbs do not. German speakers read sentences including rigid and flexible verbs and the auxiliaries "haben" (HAVE) and "sein" (BE). In an additional condition, flexible verbs were presented with a telicity-inducing prefix that led them to select BE via the verb-prefix combination. Auxiliary selection violations engendered an N400-late positivity response for both rigid and prefixed verbs, thus suggesting that the processing system sets up an auxiliary-based expectation for particular verb classes. For unprefixed flexible verbs, average ERPs did not show differential effects of auxiliary choice. However, additional mixed-effects analyses for these verbs including by-participant and by-item acceptabilities as covariates revealed modulations of the N400 and late positivity for HAVE-trials via by-item and by-participant acceptabilities. We argue that the N400 reflects the degree of match between auxiliary choice and the lexical class of the verb, while two distinct positivity effects correlate with (1) a well-formedness categorisation and (2) the degree to which individual participants engage in enriched composition in order to render a flexible (telic) verb compatible with a particular auxiliary choice (HAVE). These results indicate that the gradient behaviour of flexible verbs is not due to indeterminacy, but rather to a higher propensity for enriched composition that results in the coercion of aspectual specification.