The pyrolysis of n-propyl nitrate and tert-butyl nitrite at very low pressures (VLPP technique) is reported. For the reaction n-PrON02 A nPrO + NO2 the high-pressure rate expression at 300"K, log k, (sec-l) = 16.5 -40.0 kcal/mole/2.3 RT, is derived. The reaction tert-BuONO -% tert-BuO + N O was studied and the high-pressure parameters at 300°K are log kz(sec-') = 15.8 -39.3 kcal/mole/2.3 RT. From aSP,-1 and AS:,-, and the assumption EP1 and E-2 'v 0, we derive log kLl(M-l.sec-l) (300'K) = 9.5 and logIn contrast, the pyrolysis of methyl nitrite and methyld S nitrite afford N O and HIVO and DNO, respectively, in what appears to be a heterogeneous process. The values of k-l and k--5 in conjunction with independent measurements imply a value at 300°K for CHaO + 0 2 CHzO + HO2 of 3.5 X l o 5 M-' .sec-', which is two orders of magnitude greater than currently accepted values.