Thermal control materials of different types are being used on the spacecraft for temperature control viz. Thermal control tapes, Thermal control paints, Anodised surfaces and Optical solar reflectors. This paper covers the measurement of emittance at different temperatures on various Optical Solar Reflectors ( OSR ) both rigid and flexible types. The rigid OSRs evaluated are OCLI - USA, PPE - UK and ISRO - INDIA. The flexible OSR is from SHELDAHL - USA. The emittance of the OSRs were measured in the temperature range 223K - 373K using steady state calorimetric method. Measurements were done on a square sample with OSRs affixed on each face. The sample has a built in heater and an evacuated low temperature radiation receiver is used for the test environment.Experimental results, temperature dependence of the total hemispherical emittance relation for each of these OSRs and error estimation are presented in this paper. The results indicate that for all rigid OSRs the total hemispherical emittance increases with temperature from 223K to 323K but later drops with increase in temperature. For flexible OSR the total hemispherical emittance increases with temperature from 223K to 348K and later decreases with increase in temperature. Facilities and measurement technique used here are capable of providing the results better than ± 2.2 % in the measured temperature range.