This article examines the motivation for phonological stop assibilations, e.g. /t/ is realized as [ts], [s] or [t∫] before /i/, from the phonetic perspective. Hall & Hamann (2006) posit the following two implications: (a) Assibilation cannot be triggered by /i/ unless it is also triggered by by /j/, and (b) voiced stops cannot undergo assibilations unless voiceless ones do. In the following study we present the results of two acoustic experiments with native speakers of German and Polish which support implications (a) and (b). In our experiments we measured the friction phase after the /t d/ release before the onset of the following high front vocoid for four speakers of German and Polish. We found that the friction phase of /tj/ was significantly longer than that of /ti/, and that the friction phase of /t/ in the assibilation context is significantly longer than that of /d/. Furthermore, we unexpectedly found that the friction phase of /tj/ is significantly longer than that of /di/. An additional finding not related to the topic of the present study was that the Polish voiceless stops of the four speakers tested showed aspiration, in contrast to phonetic descriptions of these sounds as unaspirated.