We consider two classes of life distribution, V
, the members of which are defined in terms of the conditional variance σ
2(t) of the remaining lifetime of a system: a life distribution F belongs to V
if is a decreasing function and to V
if is increasing. We study closure properties of these classes under relevant reliability operations such as mixing, convolution and formation of coherent systems. We show, for example, that the class V
is not closed under convolution or mixing, and that the class V
is not closed under formation of coherent systems.