A new method for testing concrete using the initial surface absorption test (ISAT) has been developed for site use. It is based on applying a vacuum to an ISAT cap placed on the concrete surface until drying is achieved. The progress of the drying is monitored by placing indicating silica gel desiccant in the ISAT cap and observing the colour change. The method is quick, simple and practical for in situ applications. Results of comparisons with the existing in situ method in BS 1881 show that the new method is potentially more capable of producing reliable and reproducible measurements, and therefore will allow better comparison of in situ and laboratory-obtained data.