The title compound, C(12)H(14)O(6), a substituted isophthalic acid monoester which was isolated from the lichen Thamnolia vermicularis var. subuliformis, displays intra-molecular carbox-yl-meth-oxy O-H⋯O and hy-droxy-carboxyl O-H⋯O hydrogen-bonding inter-actions. The terminal methyl group of the ethyl ester is disordered over two sets of sites with occupancies of 0.599 (19) and 0.401 (19).