Abstract: This invention relates to an improved process for synthesizing carboxylic acids by the carbonylation of alcohol. An alcohol such as methanol is reacted with carbon monoxide in a liquid reaction medium containing a rhodium (Rh) catalyst stabilized with an haloacetic acid, especially trifluoroacetic acid (TFAA), along with alkyl iodide such as methyl iodide (MeI) in specified proportions. The present reaction system is not only characterized by unexpectedly high catalyst stability, but also characterized by the high reaction rate for the formation of acetic acid. The improvement resides in the employment of trifluoroacetic acid as the catalyst promoter at a temperature from 180.degree. C. to 200.degree. C. The primary advantage of the catalyst system of this invention is the enhancement of the rate of carbonylation. The other advantage of the catalyst system is that they are readily soluble and thermally stable, making them resistant to deposition.