We have carried out tourmaline-synthesis experiments in the system Na2O–CaO–MgO–Al2O3–SiO2–H2O–B2O3 at 200 MPa between 500° and 700°C under various total concentrations of Na and Ca in order to investigate the distribution of Na and Ca between tourmaline and fluid. Starting materials were oxide–hydroxide mixtures with NaCl + CaCl2 solutions of different concentrations. The main end-members of the synthetic tourmalines are dravite – uvite – “oxy-uvite” (deprotonated Ca-tourmaline). The dominant substitutions are □AlNa−1Mg−1, □Al2Ca−1Mg−2 and NaHCa−1, and their importance depends on temperature, molarity of the solution and on the ratio Ca/(Ca + Na). The Na–Ca substitution seems to be continuous at least in the steps of 25 mol.%. Unit-cell parameters, obtained from Rietveld refinement, show a systematic increase of c with increasing X-site occupancy in tourmaline and a decrease with increasing Al/(Al + Mg). Tourmalines coexist with chlorite + quartz ± plagioclase at 500° and 600°C, and with cordierite + quartz ± plagioclase at 700°C, rarely with other phases such as tremolite, enstatite or talc. The systematic response of tourmaline composition to changing Na and Ca concentrations and XCa indicates that tourmaline can be used as a tracer of fluid composition in natural systems. Furthermore, tourmaline can be used as an equilibrium check on mineral parageneses, because the Ca/(Ca + Na) value in plagioclase is the same as that in tourmaline, and Na + Ca content in cordierite is strongly correlated with X-site occupancy in tourmaline.