Several completely new guidelines representing the best methods currently available in their respective areas have been developed: The Activated Sludge Sorption Isotherm Test (835.111) is a simple and efficient method for determining the sorption potential of activated sludge for specific chemical compounds. Hydrolysis as a Function of pH and Temperature (835.2130) is a new guideline that represents an upper-tier test for hydrolysis of specific c hemical compounds for which this process is likely to be important. the Sealed Vessel CO2 Production Tests (835.3120) is a new method for determining readey, aerobic ultimate biodegradability of specif chemical compounds. The Shake-Flask Die-Away Test (835.3170) is a new guideline for OPPTS, but existed previously as ASTM Standard Test Method E 1270-89. The Sediment/Water Microcosm Biodegradation Test (835.3180) also uses natural water and sediment samples, but the purpose of this method is to determine tests compound fate in sediment/water cores collected intact from the enviornment. The Porous Pot Test (835.3220) is the last of the all-new guidelines in the 835 series.