A fit of µ a, R-branch transitions of the nearly prolate asymmetric rotor H2COâ ¯HCl (κâ â 0.99) demonstrated that even a relatively large A 0 rotational constant can be determined with useful accuracy from such a limited set of data, provided the frequencies are of sufficient accuracy. Thus the set of spectroscopic constants A 0, B 0, C 0, Î J, Î JK, δ J, Ï aa, (Ï bbâ Ï cc), Ï ab and ½{M bb+M cc} are reported for H2COâ ¯H³⁵Cl. The subsequent observation of two µ b, R-branch transitions led to a much more accurate value of A 0, but this did not lie outside the error of the preliminary value, confirming the usefulness of the initial approach. The inertial defect Î 0= 0.5724(1) u à ² demonstrates that the nuclei in H2COâ ¯H³⁵Cl are copolanar. Fits of limited numbers of µ a, R-branch transition frequencies for each of H2COâ ¯H³⁷Cl, D2COâ ¯H³⁵Cl and D2COâ ¯H³⁷Cl led to Î 0 values for the isotopomers identical within experimental error to that of H2COâ ¯H³⁵Cl.