Red lead-containing paint films have been found to lighten to basic lead (II) carbonate at high relative humidity (RH). Lightening is faster if the paint films are exposed to light. Although lightening occurs in all the binding media tested, those that most protect the pigment particles from moisture reduce the rate of lightening. In the absence of moisture or CO2, the lightening is greatly reduced. Yellow lead monoxide and samples of red lead containing some free lead (II) oxide darken rapidly. At low RH these samples remain dark, while at high RH they lighten to yield basic lead (II) carbonate. These observations help to clarify the conditions under which changes in red lead occur. Some examples of the lightening of red lead in samples from paintings are presented here. The changes seen at high RH emphasize the need to maintain paintings in appropriate environmental conditions. -- SCMRE