The aim of the proposed experiment is to use HRPD to follow the temperature dependence of the crystal structures of the double perovskites La2MgRuO6 and La2CoRuO6. This is of interest as both materials contain the Jahn-Teller active Ru4+ ion, which is prone to undergo a cooperative Jahn-Teller distortion (CJTD) and orbital ordering in oxides. Neutrons are needed to accurately follow the changes in the bond lengths and angles that are associated with a CJTD. The high resolution of HRPD is needed because of the small monoclinic distortion (angle < 90.05 degrees) of these double perovskites.