There has been little reported on the strength of phosphate-bonded investments, and in particular the role of 'special liquids' in this context is poorly understood. Using a novel test design, sequential testing of an investment diaphragm using a variable casting metal mass, a measure of the effect of 'special liquid' on strength under actual casting conditions has been obtained. The use of special liquid was found to increase substantially the strength of the four products tested. This test may provide more useful data than the conventional compressive strength test, which may give misleading information, especially if performed at room temperature.