Abstract: A method for measuring the rate of transepidermal water loss TEWL using a device for measuring TEWL which comprises: (a) a measurement chamber with a single opening at one end, which opening is adapted to be placed against a test surface of skin; (b) means to measure the density of water vapour within the measurement chamber; and (c) an air agitating means positioned within the measurement chamber; which method comprises: (i) purging the measurement chamber of the device; (ii) placing the open end of the measurement chamber against a test surface of skin; (iii) measuring the rate of change of the density of water vapour within the measurement chamber; and (iv) operating the agitating means whilst measuring changes of the density of water vapour within the measurement chamber to near uniformly mix water vapour from the test surface and air trapped in the measurement chamber.