The paper discusses a distributional restriction on the realization of cto-clause complements observed with non-agentive uses of speech act verbs. It is shown that these restrictions obtain only when the sentential complement is in the oblique position and that they are related to the argument structure of the predicate (the presence of an attitude holder argument). The paper presents the results of an experimental study using a factorial design (modelled on experimental studies of island effects), which provide evidence for the grammatical nature of the observed restriction. Several approaches to the distribution of sentential complements in generative grammar are discussed. It is argued that the observed restriction can be accounted for by the (abstract) Case requirement of sentential complements coupled with a mechanism of Case-licensing of sentential complements in oblique positions by a preposition.