FIELD: organic chemistry, chemical technology. SUBSTANCE: invention relates to preparing acetic acid and/or methyl acetate. Method is carried out by carbonylation of methanol and/or isomerization of methyl formate in liquid phase in the presence of water, solvent, homogenous catalytic system containing iridium and halogenated promoter and carbon monoxide. Catalytic system contains additionally platinum that is added to catalytic system as metallic platinum, salt, oxide or as coordination complex. Preferably, platinum is added as coordination complex with ligands taken among carbon monoxide or combination of carbon monoxide/halogen, or organonitrogen, or organophosphorus compounds. Platinum content is maintained in the concentration at least 1 mmole/l of reaction medium and atomic ratio of iridium to platinum is maintained in the range from 1 to 5. Invention provides enhancement of process output and improvement of catalytic system used. EFFECT: improved preparing method. 24 cl, 4 tbl
