Positron lifetime and angular correlation spectra were measured for 36 pure liquids, CCl4 mixtures with hexane and diethylether, and C6F6 mixtures with hexane. Apparent ortho-Ps yields, I'3, were determined as the intensity of the long-lived component in the lifetime spectra, while the apparent para-Ps yields, I'3, were obtained as the intensity of the narrowest gaussian in a three-gaussian fit to the angular correlation spectra. The ratio I'3/I1, expected to be 3, was found to be instead 2.3 (average value for 3 ethers), 2.5 (average value for 10 linear, branched, and cyclic aliphatic hydrocarbons), 3.2 (average value for 8 aromatic hydrocarbons), 2.6 (average value for 5 alcohols). Values of this ratio for various other liquids are also given. The results for the mixtures show how I'3 and I'1, vary as the Ps formation is inhibited (CCl4 mixtures) or enhanced C6F6 mixtures). The most important experimental result, namely that I '3/I1 <3 for many liquids, is discussed in detail. Although it cannot be excluded that this effect is caused by the special model (3 gaussians) used to determine I1, there are good reasons to believe the effect is real. In this case the cause might be secondary Ps reactions with other reactants in the positron spur (e.g. Ps− formation, conversion on free radicals, and Ps or Ps− oxidation by positive ions) or various processes influencing the Ps formation.