Safe Drinking Water Act Phase II regulations were promulgated on January 30, 1992 and require public water suppliers (PWSs) to begin monitoring for an additional 26 regulated and 24 unregulated synthetic organic chemicals (SOCs) in January 1993. However, in May of 1992 the EPA also signed into law the Phase V rule. This rule regulates many of the previously unregulated Phase II parameters, including dioxin. In addition, the EPA encourages PWSs to monitor for an additional 82 "priority 2" contaminants. Radical differences in costs of complying with the monitoring requirements become apparent when one considers the options for using the various approved methods, and the dates on which portions of the monitoring must be completed. The approach that states and public water supplies take with regard to waivers, sample compositing, choice of analytical methods, and integration of the phase II and V monitoring requirements will dramatically affect the final cost to the general public. Estimates for 80,000 systems to complete only the base period monitoring for organic parameters, range from $61,071,000 to $1,205,684,000. The costs of monitoring using the classical non mass spec GC procedures and the new GCMS procedures are compared. The value of the new C/MS method 525.1 to provide definitive low cost results for Phase II regulated and unregulated, as ell as Phase V regulated parameters, at the same time is presented. The dramatic cost reduction potential of a screening method for dioxin is also discussed. This chart presentation makes clear that the choice of when to monitor, as well as when to monitor for which specific parameters, can amount to substantial costs or substantial savings to the nations public water supply industry.