Ground-state rotational spectra of the three isotopomers H2CO ...(ClF)-Cl-35, H2CO ...(ClF)-Cl-37 and D2CO ...(ClF)-Cl-35 of a complex formed by formaldehyde and chlorine monofluoride were observed with a Balle-Flygare, Fourier-transform microwave spectrometer. A fast-mixing nozzle, specially modified to generate a continuous supply of H2CO from paraformaldehyde in situ close to the nozzle exit, was used to preclude the chemical reaction of H2CO and CIF. The rotational constants A(0), B-0, and C-0, centrifugal distortion constants Delta(J), Delta(JK), and delta(J), components chi(aa), chi(bb)(-) chi(cc), chi(ab) of the Cl-nuclear quadrupole tensor, and the spin-rotation coupling constant 1/2(M-bb + M-cc) were determined. Interpretation of the spectroscopic constants led to the conclusion that H2CO ... ClF is a planar complex of C-s symmetry with r(O ... Cl)=2.523(7) Angstrom, with an angle 180-phi=69.1(7)degrees between the C-2 axis of H2CO and the O ... Cl internuclear line, but with a deviation theta of the O ... Cl-F nuclei from collinearity of only 3.2(7)degrees. A family relationship between the angular geometries of several complexes B XY, where B is 2,5-dihydrofuran, oxirane, or formaldehyde, and XY is ClF or HCl, is discussed. It is shown that for a given B the angle 180-phi is the same for XY = HCl or ClF, while the nonlinearity theta of the O ... X-Y system is almost negligible for XY is ClF but significant when XY is HCl. Reasons for this family relationship and the nonlinear O ... H-Cl hydrogen bonds are discussed.