The sorption of cyclic volatile methyl siloxanes (cVMS) to organic matter has a strong influence on their fate in the aquatic environment. We report new measurements of the partition ratios between freshwater sediment organic carbon and water (K_OC) and between Aldrich humic acid dissolved organic carbon and water (K_DOC) for three cVMS, and for three polychlorinated biphenyls (PCBs) that were used as reference chemicals. Our measurements were made using a purge-and-trap method that employs benchmark chemicals to calibrate mass transfer at the air/water interface in a fugacity-based multimedia model. The measured log K_OC of octamethylcyclotetrasiloxane (D₄), decamethylcyclopentasiloxane (D₅), and dodecamethylcyclohexasiloxane (D₆) were 5.06, 6.12, and 7.07, and log K_DOC were 5.05, 6.13, and 6.79. To our knowledge, our measurements for K_OC of D₆ and K_DOC of D₄ and D₆ are the first reported. Polyparameter linear free energy relationships (PP-LFERs) derived from training sets of empirical data that did not include cVMS generally did not predict our measured partition ratios of cVMS accurately (root-mean-squared-error (RMSE) for logK_OC 0.76 and for logK_DOC 0.73). We constructed new PP-LFERs that accurately describe partition ratios for the cVMS as well as for other chemicals by including our new measurements in the existing training sets (logK_OC RMSE_cVMS: 0.09, logK_DOC RMSE_cVMS: 0.12). The PP-LFERs we have developed here should be further evaluated and perhaps recalibrated when experimental data for other siloxanes become available.