We present the calculation of the order- α s 2 correction to the deep inelastic coefficient functions C i ( x, Q 2) ( i=2, L). Its effect on the behaviour of the proton structure functions F i ( x, Q 2) for fixed target and HERA energies is studied. We investigate the dependence of the O( α s 2) QCD corrected structure functions on the mass factorization scale and the specific parametrization chosen for the parton densities. Also discussed are the implications of the higher-order QCD corrections for the extraction of the parton densities, in particular the gluon distribution function, from the data. Our analysis reveals that the O( α s 2) contributions to F 2( x, Q 2) are appreciable and can amount from +10% (large- x region) to −20% (small- x region), depending on the chosen value for Q 2 and the parton density set. In the case of F L( x, Q 2) the corrections in the small- x region can be even larger (about 45%). The large corrections in the region 0.5< x<1, which can be attributed to soft-gluon radiation, may explain the difference observed between the combined SLAC-BCDMS data and the fit based on the next-to-leading-log (NLL) approximation for the proton structure function F 2( x, Q 2).