A systematic study of metal ion doping in quantum (Q)-sized (2-4 nm) TiO2 colloids is performed by measuring their photoreactivities and the transient charge carrier recombination dynamics. The presence of metal ion dopants in the TiO2 crystalline matrix significantly influences photoreactivity, charge carrier recombination rates, and interfacial electron-transfer rates. The photoreactivities of 21 metal ion-doped colloids are quantified in terms of both the conduction band electron reduction of an electron acceptor (CCl4 dechlorination) and the valence band hole oxidation of an electron donor (CHCl3 degradation). Doping with Fe3+, MO(5+), RU(3+), OS3+, Re5+, V4+, and Rh3+ at 0.1-0.5 at. % significantly increases the photoreactivity for both oxidation and reduction while Co(3+)and Al3+ doping decreases the photoreactivity. The transient absorption signals upon laser flash photolysis (lambda(ex)= 355 nm) at lambda = 600 nm are extended up to 50 ms for Fe3+-, V4+-, Mo5+-, and Ru3+-doped TiO2 while the undoped Q-sized TiO2 shows a complete "blue electron" signal decay within 200 mu s. Co3+ and Al3+-doped TiO2 are characterized by rapid signal decays with a complete loss of absorption signals within 5 mu s. The quantum yields obtained during CW photolyses are quantitatively correlated with the measured transient absorption signals of the charge carriers. Photoreactivities are shown to increase with the relative concentration of trapped charge carriers, The photoreactivity of doped TiO2 appears to be a complex function of the dopant concentration, the energy level of dopants within the TiO2 lattice, their d electronic configuration, the distribution of dopants, the electron donor concentration, and the light intensity.