Amphiboles of predominantly tremolite composition have been synthesized at 670-800 degrees C and 200-700 MPa in the system CaO-MgO-SiO 2 -H 2 O. The tremolite crystals were large enough to be analyzed by electron microprobe. The run products were also studied by scanning electron microscopy, high resolution electron microscopy, X-ray diffraction (Rietveld analysis) and IR spectroscopy. The electron microprobe analyses resulted in average compositions between tr 87 cum 13 and tr 99 cum 1 . The cummingtonite contents determined by electron microprobe varied by up to + or -7 mol% within each run. Investigations using high resolution electron microscopy revealed that the tremolite-cummingtonite solid solutions were highly ordered and chain multiplicity faults were rare. Although the lattice parameters a, b and beta are expected to be a function of the cummingtonite content, no correlations were observed between the compositions derived by electron microprobe and the lattice parameters. The energy and absorbance of the OH stretching vibration is a function of M4 site occupancy. Therefore, the cummingtonite content was determined precisely by IR spectroscopy. At least 3 different bands at distinct energies were identified: 3674.6 cm (super -1) for the CaCa-CaCa and 3669.3 and 3672.2 cm (super -1) for the MgCa-CaCa and CaCa-MgCa configurations, respectively. The relative integral absorbances showed that all synthesized amphiboles had cummingtonite contents lower than 6 mol%. In some cases nearly pure tremolite was synthesized (tr 99 cum 1 ). This amount is considerably less than the 10 mol% cummingtonite content (tr 90 cum 10 ) often claimed for synthetic tremolite. The compositions derived by IR spectroscopy correlate with the lattice parameters. Using these correlations the lattice parameters of pure tremolite were extrapolated to be a = 9.8354(18) Aa, b = 18.0562(14) Aa, c = 5.2768(6) Aa, beta = 104.74(2) degrees , V = 906.3(2) Aa 3 .