The present disclosure is directed to a process for preparing an alkylated triiodo-substituted arylamide, such as iodixanol, the process comprising contacting a triiodo-substituted arylamide, such as 5-acetamido-N,N′-bis(2,3-dihydroxylpropyl)-2,4,6-triiodoisophthalamide, and an alkylating agent in the presence of a base and a mixed solvent system comprising a non-aqueous solvent and water, wherein the volume ratio of the non-aqueous solvent to water is greater than 1:1. The process advantageously enables the concentration of any impurities or undesirable byproduct from the reaction to be reduced, while increasing the yield of the desired reaction product.