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 A)/I(8578.7 A) and R2 = I(6161.8 A)/I(9123.6 A) as a function of electron temperature (Te) and density (Ne). The ratios are found to be very sensitive to changes in Te but not Ne for densities lower than 105 cm-3. Hence, they should, in principle, provide excellent optical Te 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 A 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 A feature is unfortunately too weak to be identified in the RR Telescopii observations, consistent with its predicted line strength.