A gaseous reductant injection control system for exhaust aftertreatment is disclosed. In one embodiment, a selective catalytic reduction (SCR) catalyst is in fluid communication with an exhaust stream generated from an engine. An oxidation catalyst (OC) is upstream of the SCR catalyst and in fluid communication with the exhaust stream. A gaseous reductant injector is upstream of the SCR catalyst and downstream of the OC and in fluid communication with the exhaust stream. A first gas sensor is upstream of the OC and a second gas sensor is downstream of the SCR catalyst. A controller receives signals representative of gas concentrations detected in the exhaust stream by the first gas sensor and the second gas sensor, and estimates concentrations of nitric oxide (NO) and nitride dioxide (NO2) in the exhaust stream therefrom.