Retention index (I) and relative retention are given for trimethylsilyl esters/ethers of approximately 170 acids and lactones. The structural categories studied are aldonic (hydroxy monocarboxylic) acids, aldaric (hydroxy dicarboxylic) acids, non-carbohydrate (mainly dicarboxylic) acids, and lactones of aldonic acids. Data are given for methyl, phenyl (50%), and trifluoropropyl (50%) silicons stationary phases. Diagrams of the retention index on the non-polar phase versus the increase in the retention index on either of the other phases are useful for qualitative analysis. The dependence of retention on structure is interpreted in terms of nonpolar and polar interactions between solute and stationary phase. Structural units of the solutes are attributed retention index increments which permit predictions of retention from structure or structure from retention. The reference retention index data can be used in temperature-programmed analysis if retentions are measured by “methylene unit” (MU) values. Differences between the I and MU concepts are discussed.