The carboxylic acids RCO2H [R = But a, Et b, 3,4,5-(MeO)3C6H2 c, 4-MeOC6H4 d, Ph e, 2,4,6-Me3C6H2 f, 2,6-Cl2C6H3 g, or 3,5-(CF3)2C6H3 h] were treated with triethylborane or with bis(9-borabicyclo[3.3.1]nonane) to give the acyloxydiethylboranes RCO2BEt2 1a-1h and the 9-acyloxy-9-borabicyclo[3.3.1]nonanes RCO2R1 (R1 = 9-borabicyclo[3.1.1]non-9-yl) 2a-2h, resp. Compds. 1a-1h are largely unassocd. in nonpolar solvents (11B NMR, IR spectroscopy). As pure liq., or in the solid state, they form equil. mixts. of monomeric and assocd. (dimeric) mols. (IR spectroscopy). Compds. 2a-2f are completely assocd. as pure liqs. and in the solid state, but only weakly assocd. in nonpolar solvents. Based on the crystal structure of dimeric 2e, it is proposed that, when assocd., derivs. of both 1 and 2 form cyclic dimers.