EN
On a Lie group NA that is a split extension of a nilpotent Lie group N by a one-parameter group of automorphisms A, the heat semigroup $μ_t$ generated by a second order subelliptic left-invariant operator $∑_{j = 0}^{m}Y_j + Y$ is considered. Under natural conditions there is a $μ̌_t$-invariant measure m on N, i.e. $μ̌_t*m = m$. Precise asymptotics of m at infinity is given for a large class of operators with Y₀,...,Yₘ generating the Lie algebra of S.