Three-photon resonance-enhanced ionization-spectra of pure CO via the A (II)-I-1 state have been obtained under a pressure range of 0.1-13 torr. It is found that the spectra can only be observed by using tightly focused laser beams, and its intensity manifests adverse pressure effect, i.e., the signal decreases with the increase in pressure for gas pressures above 1.0 torr, and almost disappears at 13 torr. Convincing experimental evidence is provided to show that the appearance of the spectra is not due to direct three-photon absorption of the fundamental laser beam, but to reabsorption of the laser-induced third harmonic generation (THG) in the CO medium. In other words, under tightly focused conditions, THG reabsorption is much stronger than direct three-photon absorption. Hence, contrary to the assertions given by previous investigators in Xe that the anomalous phenomena in three-photon excitation are caused by destructive interfer- ence, we have shown that phase matching, which greatly increases the THG, plays a major role in the three-photon resonance-enhanced ionization spectra of CO(A (II)-I-1-X (1) Sigma(+)), and accounts for both the appearance of the spectra and the adverse pressure effect of the spectral intensity. In addition, we have given an analytical treatment of the adverse pressure effect in terms of the phase-matching condition.