  The composite operator T\bar{T}, obtained from the components of the
energy-momentum tensor, enjoys a quite general characterization in
two-dimensional quantum field theory also away from criticality. We use the
form factor bootstrap supplemented by asymptotic conditions to determine its
matrix elements in the sinh-Gordon model. The results extend to the breather
sector of the sine-Gordon model and to the minimal models M_{2/(2N+3)}
perturbed by the operator phi_{1,3}.
