High resolution infrared optothermal spectroscopy has been used to study small clusters of cyanoacetylene, (HCCCN)(n), containing three or more monomer units. For the linear HCCCN trimer the fundamental of the free C-H stretch vibration has been rotationally resolved and analyzed, yielding a ground vibrational state rotational constant (B-0) of 94.031(13) MHz. In addition, an inner C-H stretch fundamental band of the linear trimer (rotationally unresolved) has been identified by observing the effect of large electric fields on the band shape. This assignment is also consistent with spectral intensity measurements as a function of molecular beam stagnation pressure. Predissociation lifetimes of the upper states of these two bands were determined from the observed homogeneous linewidths. Several other rotationally unresolved vibrational bands have also been observed. The stagnation pressure and electric field dependence of these spectra are used to estimate the cluster sizes and to make tentative structural assignments.