The crystal structure of jonesite, Ba2(K, Na) [Ti2(Si5Al)O18(H2O)](H2O)n (monoclinic, P 21/m, a = 10.618(2), b = 25.918(4), c = 8.6945(14) Å, β = 127.633(3)°, V = 1894.8(6) Å 3 , Z = 4) from the Benitoite Gem Mine, San Benito County, California has been solved by direct methods from a crystal twinned on (001) and refined to R1 = 0.045 (wR = 0.119, S = 1.028) using 3308 unique observed reflections (|Fo|≥4 σ F). The structure is based upon porous double layers of distorted Ti Φ6 octahedra (Φ = O, H2O) and TO4 tetrahedra (T = Si, Al) parallel to (010). The layers consist of two sheets of corner-sharing Ti Φ6 octahedra and Si2O7 groups each. The two adjacent sheets are linked along b by T4O12 tetrahedral rings that are disordered over two positions. The double layer has an open structure characterized by eight-membered tetrahedral rings with apertures (free diameters) of 3.37 × 3.37 and 3.33 × 3.33 Å2, for two symmetrically non-equivalent rings. K+ cations and H2O molecules are located in the pores of the double layers. Ba2+ cations are between the double layers and provide their linkage into three-dimensional structure. Jonesite is a first example of titanosilicate with porous double-layer structure.