In Islamic societies the mosque is the most important building in the community, providing a place for worship and a sense of identity. As successful communication at all levels is an important aspect of activities within the mosque, speech intelligibility is a major concern when considering acoustics within a mosque. This study investigates the acoustical characteristics of commonly built types of mosques in Saudi Arabia employing impulse response measurements. The methodology for objective field measurements is explained. Pilot field measurements were conducted in a sample mosque to characterize its acoustics and to identify the impact of active environmental control system such as air-conditioning, ceiling fans and sound reinforcement system on mosque acoustics. Several objective room-Acoustics indicators were measured and assessed with and without the operation of air-conditioning and fans. Speech intelligibility was also evaluated with and without sound reinforcement systems. The existence of acoustical deficiencies has been confirmed, and quantified.