Abstract Using low temperature X-ray powder diffraction the thermal expansion of YMn 2H 1 has been determined in the temperature region down to 4.2 K. A sharp increase in a(T) appears when cooling through the ordering temperature (T C = 200 K). Within the accuracy of our experiment no change of the cubic symmetry below T C is observable. A comparison of our results with what has been found for pure YMn 2 reveal that above T C a(T) decreases not as fast as in YMn 2 with temperature. Furthermore the step -like increase of the cubic lattice parameter at the magnetic ordering temperature is about twenty times smaller in YMn 2H 1. In our discussion we refer the deviating behaviour of YMn 2H 1 compared to YMn 2 to the suppression of the spin -fluctuation in YMn 2H 1. The hydrogen atoms cause an increase of the Mn - Mn distances and thus the formation of local Mn -moments is favoured.