The ineffectiveness of N2 as a secondary diluent in CW combustion-driven supersonic HF diffusion lasers is investigated experimentally, measuring the rotational temperatures of the DF molecules spectroscopically via the first-overtone emission and comparing the results of N2 and He systems. The results are presented graphically, plotting temperature versus the molar ratio of N2 or He to available F2. It is shown that increasing the ratio of He produces a significant drop in temperature, but increasing the N2 ratio has none. The drop in laser power and specific power (per total mass flow) with increasing N2 ratio reported by Spencer et al. (1972) and by Voignier and Regnier (1980) is confirmed. This effect can now be attributed to the inability of N2, because of its slow diffusion and low conductivity, to absorb the heat of the laser reaction.