Time-resolved measurements of infra-red emission from deuterium plasma in the theta pinch have been made at wavelengths between 70 and 250 micrometres using an indium antimonide photoconductive detector. At long wavelengths there is a sharp drop in emission near peak compression and this is attributed to an increase of the critical plasma frequency beyond that corresponding to the wave-length of observation. Thus the effect affords a means of determining the electron density. For plasma whose dimensions and impurity content are known, the absolute measurement of the radiation at short wavelengths leads to an estimate of the electron temperature, but as the emission depends only weakly on temperature the method is not of great precision.Experimental values of density and temperature show reasonable agreement with those predicted by numerical solution of the one-dimensional two-fluid hydromagnetic equations, though the comparison suggests axial loss of plasma in the practical case. The measured electron temperature is in excess of 10⁶ °K for both trapped parallel and trapped reversed magnetic field. Bremsstrahlung from impurities makes an important contribution to the radiation from reversed field discharges and appropriate allowance for this increases the estimated temperature.