Two-photon exposure is demonstrated using similar to 12 fs duration pulses at a center wavelength of 800 nm and a standard I-line photoresist. Using direct-writing with a focused Gaussian beam, developed resist features as small as 300 nm, compared with the similar to1 mum focal spot diameter, have been achieved. The experiments reveal substantial differences (similar to10(4)) in the two-photon absorption coefficient for multiple pulse trains (100 MHz) and for single pulses. A simple model involving photoexcitation of an intermediate conformation of the photoactive compound and relaxation back to the ground state can explain this observation. The resolution enhancement of root2 resulting from the two-photon absorption is demonstrated by writing two lines as close as possible to each other with both one- and two-photon absorption mechanisms.