The structure of bis(1,1,3,3-tetramethylguanidinium) dichromate was determined from powder X-ray diffraction data. The compound crystallizes in the monoclinic system (space group P21/n) with a = 10.79714 (15) Å, b = 11.75844 (16) Å, c = 8.15097 (11) Å, β = 109.5248 (6)º. The structure consists of dichromate anions (Cr2O72-) stabilized by tetramethylguanidinium cations ([H2NC(N(CH3)2)2]+ or [TMGH]+). Phase transitions of [TMGH]2Cr2O7 were determined by differential scanning calorimetry, thermal gravimetric analysis and in situ Raman spectroscopy, where the decomposition of the matrix into CrOX was found at 171-172ºC. Further heat treatment to above 400ºC resulted in formation of the thermodynamically stable Cr2O3, most likely with the [TMGH]+ cation as reductant. The catalytic activity of [TMGH]2Cr2O7 supported on TiO2 anatase in the selective catalytic reduction (SCR) of nitrogen oxide was also investigated, however only moderate activity was observed in the temperature range 100-400ºC compared to the activity of e.g. vanadia supported on titania.