This article reports the synthesis, structures and the physical properties of a homologous series of copper alkylphosphonates. Cu(O3PCH2CH3)·H2O (CuPC2) and Cu(O3PCH2CH2CH3)·H2O (CuPC3) are both monoclinic with the same space group P21/a. Hydrothermal synthesis gave blue plate-like crystals allowing single crystal resolution. The synthesis in solution of the homologous series, from phosphonic acids possessing different alkyl chain lengths (chains with n = 2, 3, 6, 10, 14 or 18 carbon atoms), spontaneously produced either monohydrated Cu[O3P−(CH2)n–1−CH3]·H2O (n = 2, 3, 6) or dehydrated materials Cu[O3P−(CH2)n–1−CH3] for n = 10, 14 and 18. The differences between the hydrated and dehydrated forms occur mainly in the inorganic slabs. The loss of the water molecule leads to a modification of the bonding scheme and the isolated [Cu2O8] dimers present in the hydrated inorganic layer are, in the dehydrated form, connected together by means of one edge. This structural change accounts for the different magnetic behaviour. In the case of dehydrated compounds, these materials were dispersed into a polyamide matrix. It has been observed that the magnetic behaviour is transferred to the polyamide composite.