Recent R-matrix calculations of electron impact excitation rates among the 3s²3p⁴ levels of Cl II are used to derive the nebular emission-line intensity ratios R₁ = I(6161.8 Å)/I(8578.7 Å) and R₂ = I(6161.8 Å)/I(9123.6 Å) as a function of electron temperature (T_e) and density (N_e). The ratios are found to be very sensitive to changes in T_e but not N_e for densities lower than 10⁵ cm^-3. Hence, they should, in principle, provide excellent optical T_e diagnostics for planetary nebulae. The observed values of R₁ and R₂ for the planetary nebulae NGC 6741 and IC 5117, measured from spectra obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope, imply temperatures in excellent agreement with those derived from other diagnostic lines formed in the same region of the nebula as [Cl II]. This provides some observational support for the accuracy of the [Cl II] line ratio calculations and hence the atomic data on which they are based. The [Cl II] 8578.7 and 9123.6 Å lines are identified for the first time (to our knowledge) in a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope. However, the 6161.8 Å feature is unfortunately too weak to be identified in the RR Telescopii observations, consistent with its predicted line strength.