A single crystal of mesolite of the chemical composition Na1.99(3) Ca2.04(1) Al6.00(4) Si8.98(3) O30.H2 O was studied by X-ray diffraction at room temperature. 40415 intensities were collected up to sinΘ/λ = 0.89 Å-1. The cell parameters are a = 18.4049(8) Å, b = 56.655(6) Å, c = 6.5443(4) Å, V = 6824 Å3, Z = 8. The refinement of 8529 unique reflections in the space group Fdd2 yielded residuals of R(F) = 0.022, RW(F) = 0.019,. GoF = 1.590. The aim of this work is to analyse the structural changes of the framework caused by increasing ratios of Ca to Na in the pores, proceeding from natrolite to mesolite to scolecite, the three zeolites with NAT topology. Mesolite can be considered to be composed of 1/3 natrolite and 2/3 scolecite. The distortion of the (Si/Al)O4 framework due to the variation of the non-framework components of the structures is reflected in different T-O distances and T-O-T angles. Their mean values differ distinctly according to the coordination of the oxygen atoms to cations or water molecules in the channels: <T-O>H = 1.657(1) Å, <T-O>Na, = 1.678(1) Å, <T-O>Ca = 1.686(1) Å; <T-O-T>H = 140.3(1)°, <T-O-T>Na = 135.6(1)°, <T-O-T>Ca = 133.4(1)°. The presentations of various structural properties (cell volume, lattice parameters, <T-O>, <T-O-T>) in dependence of the scolecite contents of the three zeolites exhibit that mesolite fits very well in between the two end members natrolite and scolecite.