The initial surface absorption test (ISAT) is examined in detail. An equation describing water absorption through an ISAT geometry circular absorbing surface is developed. It is shown that the test as currently applied involves the comparison of different parts of the absorption versus time curve for different materials. It is suggested that this results in the test being insensitive to small changes in hydraulic suction, particularly at long times. Two methods of data analysis are applied to experimental water absorption data obtained from stone and concrete materials, and it is shown that it is possible to obtain sorptivity values directly from circular source absorption data.