It is shown how the magnitude of temperature differences in both concrete bridge decks and the concrete deck slabs of composite bridge decks under various depths of surfacing may be determined by using a simple modification to the method of calculation of bridge temperatures described in TRRL Report LR 561. Empirical methods are used to estimate the magnitude of temperature differences in the steel areas of composite decks and in steel box decks. Values of effective bridge temperatures likely to co-exist with maximum positive and reversed temperature differeces are given, and the influence of depth of surfacing on minimum and maximum effective bridge temperatures is discussed. The clauses relating to temperature differences in a new British Standard Institution document on bridge loading, BS 5400, are based on the information contained in this Report, and the Department of Transport Technical Memorandom No. BEI/77 is to be revised to include this information