The structure of deuterated portlandite, Ca(OD){sub 2}, was investigated using time-of-flight neutron diffraction in the temperature range 308-643 K. Rietveld analysis reveals that with increasing temperature, the c dimension expands at a rate {approx}4.5 times larger than that for a. This anisotropy of thermal expansion is due to rapid increase in the interlayer thickness along c with increasing temperature. Fitting of the measured cell volumes yields a coefficient of thermal expansion, {alpha}={alpha} {sub 0}+{alpha} {sub 1} T, where {alpha} {sub 0}=5.966x10{sup -5} K{sup -1} and {alpha} {sub 1}=3.333x10{sup -8} K{sup -2}. On heating, hydrogen-mediated interatomic interactions within the interlayer become weakened, as reflected by increases in the interlayer D...O and D...D distances with increasing temperature. Correspondingly, the three equivalent sites over which D is disordered become further apart, suggesting a more delocalized configuration of D at high temperatures. - Graphical abstract: With increasing temperature, the Ca(OD){sub 2} structure expands {approx}4.5 times larger between the [CaO{sub 6}] octahedral layers than within the layers. Correspondingly, the D-mediated interatomic interactions become significantly weakened, and the three equivalent sites over which D is disordered become further apart, suggesting a more delocalized configuration of D at elevated temperatures.