The crystal structure of Ba 4 Ru 3 O 10 has been determined by single-crystal X-ray di4raction at room pressure. From re5nements to R โซุโฌ 0.0203 at room temperature and ambient pressure, the material is orthorhombic with space group Cmca (space group No. 64) and has lattice parameters of a โซุโฌ 5.7762(15) A s , b โซุโฌ 13.271(4) A s , and c โซุโฌ 13.083(3) A s . The unit cell thus has a volume of V โซุโฌ 1002.9(8) A s 3 and contains four formula units (Z โซุโฌ 4). Ba 4 Ru 3 O 10 is therefore of higher symmetry than the previously reported monoclinic structure based on powder X-ray data. It is isostructural with the quaternary oxides Ba 4 (Ti, Pt) 3 O 10 and Ba 4 Ir 2 AlO 10 and the ternary 6uorides Cs 4 M 3 F 10 (M โซุโฌ Mg, Co, Ni, Zn). Kinked chains of RuO 6 octahedra run along the c direction, consisting of sets of three face-sharing units joined at the corners of the end units to additional similar sets. The two distinct Ba sites show 10-fold and 11-fold coordination. Compressibilities and bulk modulus have been determined from lattice parameter variations at pressures up to 5.4 GPa. No phase transition was observed up to this pressure. Compressibility is greatest along the c axis and the bulk modulus obtained from a weighted 5t to a Vinet equation of state is 113.3(47) GPa.