The 510and 511bands of the CaCCHÃ2Π–X2Σ+transition, corresponding to the Ca–C–C bending mode, have been rotationally analyzed through cw dye laser excitation and dispersed fluorescence with a CCD array detector. The upper state is subject to Renner–Teller and spin–orbit couplings, and strongK-type resonance interactions were observed between the nearby2Δ and2Σ vibronic components. A model that invokes a full matrix treatment of these interactions was employed in a least-squares fit of a total of 708 rotational lines of the two bands, recorded with high precision. The fundamental bending frequencies have been determined as ν5= 101.394(1) and 102.940(1) cm−1for theÃ2Π andX2Σ+electronic states, respectively. The Renner–Teller parameter has been determined as ϵ5ω5= 3.528(14) cm−1. The nonadiabatic parameter for the ν5mode,gK= 0.6542(10) cm−1, is in accord with the observation that the2Δ5/2vibronic component liesabovethe normally highest κ2Σ component.