A method of estimating hydrocarbon storage in a catalytic device of an exhaust gas treatment system includes calculating an amount of hydrocarbons absorbed in the catalytic device per unit volume of exhaust gas over a period of time, calculating an amount of hydrocarbons desorbed in the catalytic device per unit volume of exhaust gas over the period of time, and calculating an amount of hydrocarbons oxidized in the catalytic device per unit volume of exhaust gas over the period of time. The amount of hydrocarbons oxidized in the catalytic device and the amount of hydrocarbons desorbed in the catalytic device are subtracted from the amount of hydrocarbons absorbed in the catalytic device to determine the amount of hydrocarbons stored in the catalytic device.