The present invention is directed to temperature modulated refractive index measurement. In accordance with the invention a method for determination of the complex temperature coefficient of the refractive index of a sample is provided, wherein the determination of the complex temperature coefficient of the refractive index of the sample is based on a refractive index measurement. Furthermore, the refractive index of the sample is measured over a period of time, wherein the temperature of the sample is modulated over said period of time and the complex temperature coefficient of the refractive index is calculated on the basis of the refractive index measurement over the period of time and the temperature modulation over the period of time. Additionally, a measurement system, in particular comprising a temperature control system and a processing system to carry out the above method, is disclosed.