In the present study, ab initio methods are used to study the NO3 hydrogen-abstraction reaction from a series of aldehydes: FCHO, CICHO, HCHO, and CH3CHO. MP2 and BH&HLYP optimizations were carried out, followed by CCSD(T) single point calculations, using the 6-311 G(d,p) basis set. Classical transition state theory (TST) is applied for the calculation of the rate constants, and tunneling corrections are considered assuming an unsymmetrical Eckart barrier. The experimental results (activation energy for the CH3CHO reaction and rate constants for the reaction of NO3 with HCHO and CH3CHO) are well reproduced at the CCSD(T)//BH&HLYP level of theory with some corrections. The same level was chosen to predict the remaining unknown kinetic parameters. At this level of calculation the reactions studied were assumed to be elementary.