The P21/m ↔ C2/m phase-transition has been studied by high-T FTIR analysis on a series of synthetic amphiboles in the Li2O–Na2O–MgO–SiO2–H2O (LNMSH) system. Spectra were collected in the T range 25–450 °C on KBr disks. All examined amphiboles have P21/m symmetry at room T. Their OH-stretching FTIR spectrum consists of two main bands at ~3740 and 3715 cm−1. At the transition temperature (Tc), these bands merge into one single absorption centred at ~3720 cm−1, and no further change is observed beyond this T. Significant modifications consisting in peak shifting and band broadening are also observed in the MIR (medium infrared) 1300–640 cm−1 region. Tc values for the different compositions were estimated based on various methods; the most reliable procedure is considered to be the fit of Landau 2-4-6 potentials using band shifts observed in the MIR region. The Tc values obtained for all samples are consistent with previous results obtained on two members of the series examined here by single-crystal or synchrotron powder HT-XRD (high-T X-ray diffraction). They correlate linearly with the aggregate cation radius at M(4) [Tc (°C) = 803–533 〈rM(4)〉; R2 = 0.97]. This work thus provides a measure of the role played by the size of the M(4) polyhedron in determining the Tc in simple chemical systems where the B-site occupancy (and geometry) is the only variable. The slope of the equation is far less steep in the LNMSH system than in cummingtonite; crystal-chemical reasons for this behaviour are discussed, and the local order between A and monovalent B cations is suggested to be the major constraint. In more complex systems, inspection of the available data shows that other factors such as the aggregate size of the strip of octahedra must be taken into account.