The addition of lithium amide (LiNH2) and ammonium chloride (NH4CI) to metathesis (exchange) reactions between gallium triiodide (GaI3) and lithium nitride (Li3N) produces crystalline gallium nitride (GaN) in seconds at ambient pressure. A specially designed rate cell incorporating multiple thermocouples enables both the reaction velocity and temperatures to be measured. Without the additives, the GaI3/Li3N reaction propagates at > 100 cm/s with a reaction temperature above 1300 K, which exceeds tile 1150 K decomposition temperature of GaN. By adding an optimal ratio of LiNH2 and NH4Cl, the reaction velocity slows to about 3 cm/s with a reaction temperature near 1200 K. Rapid heat dissipation is found to be very important in these reactions in preventing the decomposition of GaN. By using a specially designed thermal dissipation cell, the yield of GaN can be increased up to 78.8%. Applying the concepts developed in the synthesis of GaN, crystalline InN has been synthesized for the first time using solid-state metathesis reactions.