An experimental investigation has been made of the temperature effect on the angular distribution of radiation from positron annihilation in matter. The materials chosen were Teflon, fused quartz, naphthalene, and water, for which the most comprehensive studies of the long-lifetime (τ2) component and its intensity under various conditions have previously been made. The correlation between τ2 lifetime and intensity and the amount of narrow component in the angular distribution has been studied both in a single phase, and as a function of phase change. Evidence seems conclusive that the so-called temperature effect on angular distributions is in reality a density effect. The underlying cause of the narrow-component enhancement seems in part to be related simply to an increase in positronium production which follows density changes. In quartz, and in water, Teflon, and naphthalene within the present limits of error, the enhancement may be attributed to this cause alone. The narrow component in quartz, Teflon, and naphthalene is found to have a width corresponding to a center-of-mass energy at the time of annihilation of about 10 times the thermal energy for positronium. The water-ice transition shows extremely interesting properties, marked by the appearance of a very narrow and well defined peak in the ice phase. It is suggested this is due to a reduction of the zero-point energy caused by enlargement of cavities in which positronium is trapped. Some anomalies are pointed out in the excisting τ2 intensity, lifetime, and 3γ annihilation rate as a function of phase change in water.