Abstract: The present invention relates to a temperature-stable labeling reagent of formula: in which R1 represents H or an alkyl, aryl or substituted aryl group, R2 represents a detectable marker or at least two detectable markers linked together by at least one multimeric structure, L is a linking arm containing a linear succession of at least two covalent bonds and n an integer equal to 0 or 1, R3 and R4 represent independently of each other: H, NO2, Cl, Br, F, I, R2-(L)n-Y—X—, OR, SR, NR2, R, NHCOR, CONHR, COOR with R=alkyl or aryl, A is a linking arm containing at least one covalent double bond allowing conjugation of the diazo functional group with the aromatic ring and u is an integer between 0 and 2, preferably equal to 0 or 1, and —Y—X— represents —CONH—, —NHCO—, —CH2O—, —CH2S—.