It is an object of the present invention to provide a beneficial composition in order to detoxify the harmful compound containing arsenic etc. effectively and systematically and a method for detoxifying a harmful compound by using the composition. The composition for the alkylation according to the present invention is characterized in that the composition contains a cobalt complex. The method of detoxifying the harmful compound according to the present invention is characterized in that a harmful compound containing at least one element selected from the groups comprising arsenic, antimony and selenium is detoxified by the alkylation of the harmful compound, in the presence of the composition according to the present invention.