The quantum yield, phi, of methyl orange photocatalytic oxidation on TiO2 nanoparticles under periodic illumination drops from phi(tauL)-->(0) to phi(tauL)-->(infinity) in two well-resolved steps at increasingly longer bright intervals, tau(L), when they are separated by sufficiently long dark periods, tau(D). The {tau(L1) < tau(L2)} values at which the inflections occur depend exponentially, but with opposite trends, on the solution pH. The condition tau(D) much greater than tau(L) ensures charge sparsity, leads to a stochastic kinetics regime in which carrier recombination is minimized, and lets carriers manifest their dissimilar redox reactivities. The more reactive intermediates in acid (basic) media are ascribed to the photogenerated holes (electrons), the crossover occurring ca. pH 8. We found that phi(tauL)-->(0) and phi(tauL)-->(infinity) coincide with the phi values measured under steady illumination at (gammaI(a)) and I-a photon absorption rates, respectively, gamma = tau(L)/(tau(L) + tau(D)) being the duty cycle. The similar phi vs tau(L) behaviors observed for methyl orange and formate are traceable to the dynamics of interfacial species. The photochemically relevant intermediates persist longer than a few milliseconds under typical conditions, i.e., several orders of magnitude longer that the plethora of transient spectroscopy signals (half-lives down to < 1 ps) previously associated with these species.