Self-diffusion measurements made by steady or pulsed field gradient spin echo NMR are not absolute and the magnetic field gradients employed must normally be determined by calibration with liquids with known self-diffusion coefficients. The primary calibrant is water, with self-diffusion coefficient values having been extrapolated from the tracer diffusion of HDO and of HTO in ordinary water by Mills,(1) with a relative standard uncertainty of 0.2 %. This and other liquids presently used for calibration all have low viscosities. Current work on ionic liquids, which are generally quite viscous, suggests there may be problems with the pulsed field gradient (PGSE) techniques usually employed as results dependent on the time interval between gradient pulses have been reported by Hayamizu et al.(2) In this work, self-diffusion coefficients, obtained by a steady gradient (SG) technique, are reported for the viscous molecular liquids squalane, ethylhexyl benzoate, and bis(ethylhexyl) phthalate (DEHP), and it is suggested that these substances may be suitable secondary reference materials for the calibration of spin echo NMR apparatus when self-diffusion in viscous liquids is to be measured. New PGSE measurements for squalane and DEHP are in good agreement with the SG results. We also report on systematic errors found in the secondary calibration data of Holz et al.(3) for cyclohexane, n-dodecane, dirnethyl sulfoxide, and pentan-l-ol (though not for 1,4-dioxane) and suggest toluene in their place as a more convenient low-viscosity calibrant that is also suitable for low temperature work.