A microcapsule comprising a hydrophobic core within a polymeric shell, in which the core comprises: (a) 10 to 65% by weight of a hydrocarbon liquid or hydrocarbon wax containing hydrocarbon molecules of between 10 and 24 carbon chain length; and (b) 35 to 90% by weight of an aliphatic acid containing at least 6 carbon atoms, based on the total weight of the core. The microcapsules are suitable for use in heat storage systems and especially in heat transfer systems were it is normally necessary for the density of the microcapsules and the carrier fluid to be essentially the same.