We characterized the photoluminescence (PL) decay of 15 different, solubilized single-walled carbon nanotubes with tube diameters that ranged from 0.7 to 1.1 nm using time-correlated single photon counting. Each nanotube species was excited resonantly at the second excited state, E2, and PL was detected at the lowest energy exciton emission, E1. In a 10 ns window, the PL decays were described well by a biexponential fitting function with two characteristic time constants, suggesting that at least two kinetically distinct relaxation processes were observed. The dominant decay component increased from 60 to 200 ps with increasing tube diameter, while the lesser component, which contributed up to 8% of the total decay, increased from 200 ps to 4.8 ns. The observation of the second, longer decay time component is examined in terms of two possible models: an extrinsic behavior that implicates sample inhomogeneity and an intrinsic process associated with interconversion between kinetically distinct bright and dark exciton states. A common conclusion from both models is that nonradiative decay controls the PL decay by a process that is diameter dependent.