One of the advantages of the membrane conductivity measurement system employed in the Sievers 900 Series TOC Analyzers, compared to other TOC analyzers, is the excellent stability of the calibration. To illustrate this, a series of tests were conducted on three 900 Series analyzers over the course of a 14-month period and the results are summarized in this report . The 900 Analyzers used were a laboratory model (S/N 59) and two portable models (S/N405 and 531), all equipped with autosamplers. The analyzers were calibrated using the multi-point calibration protocol that includes calibration of the TC and IC channels at 1, 5, 10, 25 and 50 ppm using Sievers Standards of potassium hydrogen phthalate (TC) and sodium carbonate (IC). The analyzer’s TOC offset, an adjustment to match the TC and IC channels, was performed using air-saturated water. Two of the analyzers (S/N 59 and 531) were calibrated in August 2005, and S/N 405 was calibrated in May 2005.