The opalescence of ethylene has been investigated under various conditions in the region of the critical temperature. The opalescence of critical fillings increases as the temperature is lowered and passes through a maximum at the critical dispersion temperature Ts. When the system is heated the value depends upon the thermal history, both below and above Ts. Two types of time lag curve are observed in the irreversible region, one in which the opalescence passes through a minimum and then a maximum value, and the other through a minimum only. It is believed on the basis of previous qualitative studies that violent shaking would result in reversibility with respect to changes in temperature. Once the apparent phase discontinuity has disappeared the opalescence becomes reversible as long as condensation is prevented. The opalescence of critical fillings depends upon the density and appears to be a maximum when the two phases are present in equal volumes at Ts. Air and oleic acid present in small quantities depress Ts but do not appear to change the variation of opalescence with (T − Ts) when T > Ts. These observations appear to disagree with the classical Einstein–Smoluchowski theory based on light scattering resulting from statistical fluctuations in density. This theory predicts a maximum opalescence at the classical critical temperature, which in the case of ethylene is 0.7 °C. higher than Ts. If, however, the more recent views of the nature of the P − V isotherms near the critical point are taken into account, some of the conflict disappears.