Hydrochloric acid solutions were reacted with limestone core plugs using an annular flow reactor at acid-fracturing conditions. The obtained reaction rates were correlated with theory using a correlation that was recently verified to be suitable for correlating HCI reaction rate with mass transfer parameters. Applying the correlation to the present data produced effective diffusion coefficients for HCI from the different acid solutions at acid-fracturing conditions. The results showed that the diffusion coefficient of an acid solution made in distilled water was larger than the diffusion coefficient of an acid solution made in synthetic North Sea water. The diffusion coefficient of a retarded acid was found to be an order of magnitude smaller than the diffusion coefficient of neat acid.