Abstract: The present invention provides a reagent composition for measurement of ionic strength of liquid samples (especially for measurement of specific gravity of urine), which contains at least one phosphoric diester, at least one pH buffering agent, and at least one pH indicator. The present invention also provides a test tool containing the reagent composition which may be a test strip made by impregnation, coating, or printing of the composition. The reagent composition and the test tool are convenient and less susceptible to pH and temperature of the samples, measurement timing, etc. The compounds shown by the following general formula (1): ##STR1## wherein R.sub.1 and R.sub.2 in the formula (1) represent straight or branched alkyl groups containing 4-20 carbon atoms, phenyl groups which may be substituted, benzyl groups which may be substituted, or groups containing polyalkylene glycol chains, are preferable as a phosphoric diester.