On May 14, 1999, we promulgated Method 1664, Revision A: N-Hexane Extractable Material (HEM; Oil and Grease) and Silica Gel Treated N-Hexane Extractable Material (SGT-HEM; Non-polar Material) by Extraction and Gravimetry (Method 1664A; the Method) at 40 CFR part 136 for use in EPA's CWA monitoring programs and at 40 CFR part 260 for use in EPA's RCRA monitoring programs. The purpose of this Guidance is to assist you in the application of Method 1664A to wastewaters and wastes, and to address potential differences between results that could be obtained using Method 1664A and approved methods that employ CFC-113 as the extracting solvent. This Guidance also further explains the flexibility allowed within the scope of Method 1664A, particularly the use of solid-phase extraction (SPE), and answers frequently asked questions (FAQs) concerning Method 1664A. This Guidance is specifically written to provide you with procedures for side-by-side testing of specific discharges (when necessary), and with clarification on the use of solid-phase extraction. To help in this process, the following topics are addressed: (Section 2) discusses differences in results produced by EPA Method 1664A and CFC-based methods and how to determine if results are equivalent or if determining a conversion factor is necessary; (Section 3) discusses use of solid-phase extraction (SPE) and how to determine if results produced with SPE are equivalent to those produced by liquid-liquid extraction (LLE) using CFC-113 (LLE/CFC-113) or Method 1664A (LLE/n-hexane); (Section 4) discusses flexibility in EPA Method 1664A; (Section 5) presents responses to specific questions raised by the regulated community regarding the use of Method 1664A.