The durations of hesitation devices such as the sounds /α, ϵ, æ, r, ∂, m/, also called filled pauses, were measured and compared with the durations of silent hesitations or unfilled pauses. Their individual consistency and psychological significance were also investigated and the relation to uncertainty of filled pauses and unfilled pauses respectively was compared. It appears that under certain conditions of speech production the two hesitation phenomena reflect different internal processes.