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The effects of short-term organic shock loads on the performance of a laboratory scale two-stage activated sludge (AS)-biofilm reactor working at 6h HRT and treating medium strength domestic wastewater was studied by increasing the influent chemical oxygen demand (COD) to 2–4 times the normal values. Each shock load was applied for a period of 6h, after which normal loading conditions were resumed. The maximum effluent COD concentration obtained was 169, 169, 250 and 617mg/L under the shock loads of 808, 1170, 1358 and 1900mg COD/L, respectively. The COD removal rate increased with increasing effective OLR. The system recovered quickly from shock loads; recovery time proportional to the magnitude of shock loads. The optimum loading rate of the reactor was found at effective OLR of 170mg COD/m³/h. Shock loads changed the dominant bacterial type in the reactor from Gram-positive rods …