The HYSCAN II instrument has been used to measure the hydrogen content in unmodified and modified hypoeutectic Al-10wt%Si alloys. It is shown that Sr addition markedly increases the hydrogen content and accelerates the gassing rate of the Al-Si melts. At the beginning, the hydrogen content constantly increases, and at a certain time, it reaches a peak. After this, it begins to decrease and tends to be close to the initial hydrogen level. In contrast in unmodified melts, hydrogen content decreases with time, finally to a stable level. Moreover, without modification, the porosities are found to be present as a long, fissured, irregular shape. Tn modified samples, however, the porosities are broad, rounded and regular. The mechanism of the effect of Sr-modification has also been studied.