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Complexes of the structural isomers, HCNO (fulminic acid) and HNCO (isocyanic acid), with argon have been studied employing pulsed-beam Fourier-transform microwave spectrometers. Both complexes exhibit predominately b-type rotational transitions. Less intense a-type lines were also observed for the argon, isocyanic add complex. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}4N$ hyperfine structure was resolved for Ar-HCNO. Ar-DCNG, Ar-HNCO and Ar-DNCO. The spectra wore fit to a semirigid asymmetric top Hamiltonian with one quadrupolar coupling nucleus. The spectroscopic constants obtained from the fits are: Ar-HCNO (in MHz), A = 11807.775(2), B = 2010.593(9), C = 1705.380(8), $\Delta J=0.01581(4),\Delta JK=0.2366(2).\Delta K=-0.0824(4),\delta J=0.002451(6),\delta K=0.177(4),\chi aa=-0.130(6)and(\chi bb-\chi cc)=0.358(8);Ar-HNCO(inMHz),A=11224.568(1),B=1937.6358(7),C=1638-6614(4).\Delta J=0.01470(1),\Delta JK=0.3888(2),\Delta K=-0.3140(6),\delta J=0.002464(4),\chi aa=0.946(4),and(\chi bb-\chi cc)=3.101(7).$ Ar-HCNO is approximately T-shaped and the moments of inertia give an argon fulminic acid center of mass separation of $Rcm$ = 3.49{\AA} and an angle of $80.2\circ$ between $Rcm$ and the molecular axis of fulminic add. The Ar-DCNO isotopic data indicate the oxygen is tilted toward argon. A similar structure is determined for Ar-HNCO From the moments of inertia of the normal isotopomer. In this complex, $Rcm$ = 3.56{\AA} and the analogous angle between $Rcm$ and the a inertial axis of isocyanic acid is $83.3\circ$. The Ar-DNCQ isotopic data show the hydrogen is directed toward the argon atom and also pro vide support for a planar structure.