Dedicated to Professor Caroline Röhr on her anniversaryTwo polymorphs of burgundy-red OsP 2 O 7 , with cubic (ZrP 2 O 7 structure type, Z = 4, a = 7.8276(4) Å, 3 × 3 × 3 superstructure with a' = 3a = 23.484(2) Å) and triclinic symmetry (ruby-red crystals, single-crystal X-ray structure determination, GeP 2 O 7 structure type, P � 1 (no. 2), Z = 2, a = 4.8072(7) Å, b = 6.8993(13) Å, c = 8.050(1) Å, α = 91.82(2)°, β = 92.92(1)°, γ = 107.42(2)°, 98 parameters, R 1 = 0.045, wR 2 = 0.144, 3006 unique reflections with j F o j > 4σ(F o )) have been obtained reducing OsO 4 by phosphorus in sealed silica ampoules followed by subsequent chemical vapour transport. Reaction with the wall led to mixed Si IV /Os IV silicophosphates (Os 1À x Si x ) 3 [Si 2 O(PO 4 ) 6 ] {x = 0.84, pale orange, Si IV 3 [Si 2 O(PO 4 ) 6 ] structure type, R � 3, Z = 3, a = 7.893(1) Å, c = 24.409(5) Å, 59 parameters, R 1 = 0.058, wR 2 = 0.160, 1311 unique reflections with 1033 j F o j > 4σ(F o ); x = 0.47, orange, Ge IV 3 [Si 2 O(PO 4 ) 6 ] structure type, P � 31c, Z = 2, a = 7.9979(3) Å, c = 16.4625(5) Å, 59 parameters, R 1 = 0.040, wR 2 = 0.094, 902 unique reflections with 749 j F o j > 4σ(F o )}. Both structures were refined from single-crystal data. Above 25 K triclinic OsP 2 O 7 shows temperature independent paramagnetism with a strong temperature dependence of m=m B pointing to 3 T 1g as electronic ground state for Os 4 + (d 4 ) with strong spin-orbit coupling leading to J = 0 and the nonmagnetic ground state of the octahedral [Os IV O 6 ] chromophore.. Optical spectra of both polymorphs of OsP 2 O 7 show similar combinations of intense sharp and broad absorption bands typical for d n electronic systems systems with both, spin-orbit coupling and ligand field splitting, being strong. The observations are consistent with the ligand-field parameters Δ o = 20000 cm À 1 , B = 704 cm À 1 , and ζ = 3454 cm À 1 for the octahedral [Os IV O 6 ] chromophore.