Describes a procedure for collection of decay rate data from stone samples exposed in actual environmental conditions. This provides precise weekly decay rate data. Data were collected for 16 consecutive months for 200-year-old weathered and freshly quarried Portland stone. Problems with the use of an acrylic sheet as a control were encountered and overcome. It is suggested that glass is a better control surface. Decay rates of new and old stone differed, with new stone typically being 60-70 mg/m²/day. Close agreement was found between new stone replicates. Rates for old stones were much more variable and ranged from 90-270 mg/m²/day. Differences were also found in the relative contributions of solution and particulate loss to total loss between the two types of stone.