Laboratory experiments were performed to measure the ratio of rate constants for reactions (R3): NO3+NO→2 NO2 and (R4): NO2+NO3→NO+NO2+O2. This was accomplished through direct measurement of the nitric oxide and nitrogen dioxide concentrations in an N2O5/NO2/N2 mixture over the temperature range from 273 to 313 K. The measured ratio is represented by the expressions k3/k4=387 exp(-1375/T). The result, when combined with measurements of k3 in order to determine k4, is in reasonable agreement with other recent results from this laboratory; however, it is approximately a factor of 2 smaller than earlier reports. The experimental details are reported along with possible atmospheric implications of this result. ¿ American Geophysical Union 1990