The elastic modulus of a honeycomb sandwich core under transverse compression should be simply (ρE/ρ0), where ρ is the density of the honeycomb, ρ0 that of the material of the honeycomb and E the Young's modulus of the material. However when honeycomb sandwiches are tested under compression between flat platens it is common experience that the deformations recorded are markedly greater than those predicted from the theoretical modulus. In order to throw light on this discrepancy comparative tests have been made on three samples, two of normal thicknesses, namely in and in, and the third 8 in thick.