In this paper I have introduced one case of modification of articulation at the word-boundary, i. e. the assimilation, which regularly takes place in Dutch, but is impeded in Czech. In Dutch the assimilation of [s] in front of [j] is the result of coarticulation, applied in order to reduce the articulatory effort. It is, however, not an isolated phenomenon, but it is connected with the not clearly defined phonological and phonetic opposition of [s] and [š]. In Czech, despite the presumption that the coarticulation of the [s] in front of [j] also is possible, it is not audible for the speakers as modification, and - most importantly - it does not yield assimilation of the former to the latter, i. e. the realization of [š]. In our opinion the constraints are complex, in the first place we should point to the distinctive role of the /s/ and /š/ in the whole system. The perceptual segmentation in Czech, moreover, is ruled by the distribution of word-stress at the initial syllable of the word (after a pause); this brings about the hyperarticulation of the syllable, so that no need is felt to reduce the articulation. Because of the typological difference between Czech and Dutch, as respectively inflective and not-inflective languages, the phonetic phenomena at the word-end can be very significant and should be examined further.