The effect of pH on inhibition and enhancement of luminol–H₂O₂–CO² chemiluminescence (CL) by 18 phenolic compounds and 20 amino acids was studied. It was found that most of the tested compounds showed an inhibiting effect at lower pH and an enhancing effect at higher pH. At a midrange pH, for some phenolic compounds with two ortho-position –OH, both an inhibiting and an enhancing peak were simultaneously observed. UV–visible spectra of the tested phenolic compounds at different pH values were studied. The mechanism for CL inhibition and enhancement was proposed. It is likely that the competition of the –OH or the –NH₂ group and other reducing groups in the molecules with luminol for O₂^·− led to the CL inhibition. A reaction of –COO⁻ and quinone or ketone formed by phenolic compounds at higher pH via deprotonation with O₂^·− also resulted in the CL enhancement.