Our laboratory, along with several others, has been engaged in a project to determine the suitability of diisodecylphthalate (DIDP) as a standard reference material of viscosity for industrial purposes. As a part of that project, we have undertaken a study of the effects of surface tension and of impurities, of that liquid on viscosity measurements with routine, suspended level capillary instruments. For the former purpose, a set of surface tension measurements is reported here for DIDP and two reference standard mineral oils. In particular, surface tension data for DIDP, obtained using a pendant-drop shape-analysis method, at temperatures from 288K to 308K are presented.The present study suggests that surface tension effects upon viscosity measurements with routine capillary viscometers can be important enough to demand that the surface tension of the reference fluid(s) used for their calibration be specified. It is also evident that the surface tension of materials used for calibration may be relevant to their selection if errors arising from surface tension are to be avoided. In order to fulfil these requirements, the surface tension of the reference material should be included in the definition of the standard.The effect of the water content on the viscosity of a sample of DIDP is assessed as a part of the process of determining its suitability as a standard reference material.