A seat belt system (10) comprises a seat belt (15) and a tongue assembly (22). A buckle (26) engageable with the tongue assembly (22) secures the system (10) in a buckled condition in which the belt (15) is divided into a torso portion (28) and a lap portion (29). The system (10) also includes an inflatable device (100) comprising an inflatable portion (98) of the belt (15). An inflation fluid source (40) provides inflation fluid to the inflatable device (100) only when the tongue assembly (22) is engaged with the buckle (26). A cinch mechanism (130) has a cinched condition resisting movement of the belt (15) through the tongue assembly (22). The inflatable device (100), when inflated, causes the cinch mechanism (130) to move to the released condition and causes the torso portion (28) and the lap portion (29) of the belt (15) to be tensioned about the vehicle occupant.