The angular correlation of the two-photon annihilation of positrons annihilating in gases at high pressures demonstrates the two modes of decay-(1) from a bound state, positronium, and (2) from an unbound state-in all gases tested that have low-lying energy states. The increase in the narrow component in Ar, plus suitable thermalizing gases, due to the application of a dc magnetic field obeys the theoretical increase due to the mixing of the 1 /sup 1/S and 1 / sup 3/S m = 0 states. The increase in positronium formation due to the application of a dc electric field is observed. Preliminary evidence for the existence of n = 2 states of positronium is seen. (auth)