A compound of the formula I: wherein R is H or —N═N-2-carboxyphenyl; A is (CH2)n or —CH═CH—, wherein n is an integer from 0 to 10, or A may also be —CH(COOH)— when R is —N═N-2-carboxyphenyl; and X is a radical selected from the group consisting of: (i) Cl; (ii) COOR1, wherein R1 is p-nitrophenyl or N-succinimidyl; (iii) CONH—NHR2, wherein R2 is H, COO(t-butyl) or COObenzyl; (iv) CONH—[B]—NHR3, wherein R3 is H, COOR1, or CO—[B′]—maleimido, wherein R1 is t-butyl, p-nitrophenyl or N-succinimidyl, and B and B′, the same or different, are (CH2)n wherein n is an integer from 2 to 10; (v) CONH—[B]—COOR4, wherein R4 is H, C1-C8 alkyl, N-succinimidyl; (vi) CONH—[B]—OH; (vii) CONH—[B]—CONH—NHR2, wherein R2 is H, COO(t-butyl) or COObenzyl; and (viii) NHR2, wherein R2 is H, COO(t-butyl) or COObenzyl, when A is —CH(COOH)— and R is —N═N-2-carboxyphenyl. The 4′-hydroxyazobenzene-2-carboxylic acid (HABA) compounds are novel reagents for labeling, isolation and detection of biological molecules.