1 An improved method for estimating the zwitterion constants of phenolic amines is described which involves the exploratory least-squares fit of absorbance (at a suitable wavelength) to pH, starting with estimates of pK1 and pK2 obtained electrometrically. 2 With the method it is possible to see that hydroxyamphetamine (alpha-methyltyramine) has a higher zwitterion constant than tyramine and the zwitterion constants of both compounds are lower at 37 degree C than at 25 degree C. 3 The zwitterion constant of morphine is not reduced by raising the temperature from 25 degree to 37 degree C and the effect of temperature is much greater in compounds with a primary or secondary amino group than with those containing a tertiary amino group. Some zwitterions may be stabilized by hydration and their formation will be reduced by a rise in temperature which will break up water structure. 4 From electrometric titrations with (+)-tubocurarine chloride in 0.1 M NaCl estimates of pK1, pK2 and pK3 were 7.6, 8.65 and 9.65 at 25 degree C and 7.4, 8.6 and 9.7 at 37 degree C, compared with 7.8, 8.85 and 9.75 given by Perrin (1980). However, the effects of pH on absorbance show that the phenolic groups lose a proton before the ammonium group so there is extensive zwitterion formation which is probably greater at 25 degree than at 37 degree C. the p-phenolic group (position 13) probably ionizes first with the phenate form stabilized by hydration involving water molecules and the protonated form of the (1-) ammonium group.