When the propargyl radical, HCCCH(2), and O(2) are codeposited onto a cold argon matrix, a chemical reaction ensues; infrared absorption spectra reveal the formation of the propargyl peroxyl radical: HC=C-CH(2) X (2)B(1) + O(2) --> trans-HC=C-CH(2)OO X (2)A. We do not observe the isomeric adduct, CH(2)=C=CHOO X (2)A''. The propargyl radicals are produced by a hyperthermal nozzle while a second nozzle alternately fires bursts of O(2)/Ar at the 20 K matrix. The absorption spectra of the radicals are measured using a Fourier transform infrared spectrometer. We observe 13 of the 18 fundamental infrared bands of the propargyl peroxyl radical in an Ar matrix at 20 K. The experimental frequencies (cm(-1)) of trans-HC[triple bond]C-CH(2)OO X (2)A'' are assigned. The a' modes are nu(1) = 3326, nu(2) = 2960, nu(3) = 2148, nu(4) = 1440, nu(5) = 1338, nu(6) = 1127, nu(7) = 982, nu(8) = 928, nu(9) = 684, and nu(10) = 499 cm(-1), while the a'' modes are nu(14) = 1218, nu(15) = 972, and nu(16) = 637 cm(-1). Linear dichroism spectra were measured with photo-oriented HCCCH(2)OO radical samples to establish the experimental polarizations of several vibrational bands. The experimental frequencies (nu) for the propargyl peroxyl radical are compared to the anharmonic frequencies (upsilon) resulting from electronic structure calculations. We have used CBS-QB3 electronic structure calculations to estimate the peroxyl bond energies: DeltaH(298)(trans-HC[triple bond]CCH(2)-OO --> CH(2)CCH X (2)B(1) + O(2)) = 19 +/- 1 kcal mol(-1) and DeltaH(298)(trans-CH(2)=C=CH-OO --> CH(2)CCH X (2)B(1) + O(2)) = 21 +/- 1 kcal mol(-1). The experimental thermochemistry for C(3)H(3) reacting with oxygen has been reanalyzed as Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)=C=O + HCO) = -83 +/- 3 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(3)CO + CO) = -111 +/- 3 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)CHO + CO) = -106 +/- 4 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> HCHO + HCCO) = -67 +/- 4 kcal mol(-1); Delta(rxn)H(298)(HCCCH(2) + O(2) --> CH(2)CH + CO(2)) = -105 +/- 3 kcal mol(-1).