In this study, three mixtures of methylyltriphenylphosphonium bromide (MTPPB) as hydrogen bond acceptor (HBA) and N-methyl diethanolamine (MDEA) as hydrogen bond donor (HBD) component was used to prepare transition temperature mixtures (TTMs) into different mole ratios of 1:7, 1:10 and 1:16 HBA/HBD. Two important physicochemical properties of TTMs such as density and refractive index were investigated at the atmospheric pressure and temperature ranges of (293.15�353.15) K and (293.15�343.15) K, respectively. The experimental density data were used to derive the molar volume, molecular volume, lattice energy and isobaric thermal expansion coefficients. With the help of experimental refractive index data, the electronic polarization, molar refraction, and free volume were calculated at the whole temperatures. Several empirical equations were used to correlate refractive indices such as an empirical equation and one-parameter equations (Dale�Gladstone, Eykman, Lorentz�Lorenz, Newton, Arago�Biot, and Oster). Finally, the response surface methodology (RSM) was applied to evaluate the effects of two main factors such as temperature and mole ratio on the density and refractive index of TTMs. The results revealed that the molar ratio has almost a higher effect on the studied properties than temperature. © 2018