Abstract: Four 3,4-dihydroquinoxalin-2-(1H)-one derivatives (details given) were evaluated as fluorogenic substrates for horseradish peroxidase and its mimetic enzyme haemin. The performances of these substrates were compared to those of commercially available substrates, namely, p-hydroxyphenylacetic acid (p-HPA), p-hydroxyphenylpropionic acid (p-HPPA), homovanillic acid (HVA) and tyramine. The evaluations were performed by a FIA method in which a mixture, formed by merging substrate and buffered enzyme streams, was subsequently merged with a water carrier stream containing 150 µL H2O2. The reaction was monitored by fluorimetry at the optimum wavelength for each substrate. The substrates p-HPPA, p-HPA, NN'-dicyanomethyl-o-phenylenediamine (DCM-OPA) and 3-methyl-3,4-dihydroquinoxalin-2-(1H)-one (MDHQ) exhibited comparable performances; detection limits were at the nM level for H2O2. DCM-OPA had a better stability than MDHQ but both were stable for at least one month in a refrigerator.