Novel acetic acid derivatives of the formula: ##STR1## wherein X is oxygen or sulfur; R.sub.1 is hydrogen or a lower alkyl; R.sub.2 is hydroxy, an alkoxy, an alkenyloxy, a cycloalkyloxy, an aryloxy, a substituted aryloxy, an aryl-lower alkoxy, a substituted aryl-lower alkoxy, an aryl-lower alkenyloxy, a substituted aryl-lower alkenyloxy, an .omega.-disubstituted amino-lower alkoxy, a lower alkoxycrbonyl-lower alkoxy, a lower alkoxy-lower alkoxy, a hydroxy-lower alkoxy-lower alkoxy, an acyl-lower alkoxy, or a group ##STR2## wherein R.sub.3 is hydrogen, hydroxy, a lower alkyl, an aryl-lower alkyl, a hydroxy-lower alkyl, or a carboxy-lower alkyl, R.sub.4 is hydrogen, a lower alkyl or a hydroxy-lower alkyl, or the R.sub.3 and R.sub.4 may combine together with the nitrogen atom to which they are joined to form a heterocyclic group; and the group --CH(R.sub.1)COR.sub.2 is joined to the carbon atom at 2- or 3-position, and pharmaceutically acceptable salts thereof, processes for their preparation, and pharmaceutical compositions containing these compounds. They have excellent anti-inflammatory, analgesic and antipyretic activities with weak ulcerogenicity in the gastrointestinal tract and toxicity.