Different mechanisms are proposed to account formation of monodisperse nanocrystals in literature, each of which is usually proposed to explain one set of experimental observations. Here, a general model based on mass conservation is developed to fully describe all possible channels including free growth by direct incorporation of the monomers converted from the precursors, growth by dissolution of a portion of the regular nanocrystals in solution, and growth by dissolution of the clusters in solution. The new model provides convenient yet quantitative methods to determine the channel ratios at a given time. Experimentally, an automated microreactor system is developed and applied for synthesis of monodisperse CdS nanocrystals, which is coupled with liquid-phase Fourier transform infrared and UV-vis measurements to, respectively, determine precursor conversion and size/concentration of nanocrystals with high reproducibility (<1%) and proper time resolution (<1 s). Different from the most-accepted model for formation of monodisperse nanocrystals, a burst of nucleation followed by growth of all nuclei by direct incorporation of the monomers converted from the precursors (or "focusing of size distribution"), all three basic channels are found to coexist during growth of monodisperse CdS nanocrystals. While the new theory and experimental methods are applied to study growth of monodisperse nanocrystals, they can be extended to offer a full kinetic picture for formation of colloidal nanocrystals.