A study of lithium and sodium insertion in W3Nb14O44 a block structure type phase, has been carried out. During the electrochemical insertion, the formation of several solid solution regions of general formula LixW3Nb14O44 (0 less than or equal to x less than or equal to 28) and NayW3Nb14O44 (0 less than or equal to y less than or equal to 0.5), have been observed. Structural changes originated by lithium insertion in the parent oxide, have been evaluated by X-ray powder diffraction.