The (2,4-pentanedionato)carbonyltriphenylphosphinerhodium [Rh(acac)(CO)(x)(PPh3)(y)], x = y = 1] precursor (I) of the active catalyst forms (x = y = 2; x + y = 4) was prepared in 3 steps: (i) RhCl(3)nH(2)O (n = 2-3) was carbonylated in hexane as solvent with CO at 78degreesC for 3.5 h in 97% yield (Fig. 1); (ii) the Cl- ligand was exchanged (with BaCO3 + 2,4-pentanedione) for a 2,4-pentanedionate (= acac) ligand at 45-60degreesC in 0.25-1.5 h; BaCO3 was filtered off, hexane removed, and Rh(acac)(CO)(2) dried in N-2 at 40degreesC; (iii) the CO was exchanged for PPh3 at 50degreesC for 0.5 h in hexane and I was collected by filtration, washed with hexane, and dried at 60degreesC. The structure of I was confirmed by FTIR, H-1 NMR and C-13 NMR (CDCl3, 75 MHz) spectra. Rh in I was determined by AAS. At [Rh(acac)(CO)(2)]:PPh3 = 1:1-1:1.05 (by moles), the start (T-s1) and end m. ps. (T-s2) were close to those of a commercial reference I (ROPAC). Two I specimens prepared and used to make C3H6 + H-2 + CO (1.13:1.17:1 by moles) react over 0.021-0.023% Rh and to convert butanal into 2-methylpropanal were found to be superior by 9.8 and 13.5% to the ROPAC. Oxo syntheses are reviewed.