A new class of substituted 2,3-naphthalenedicarboxaldehydes (NDA) is disclosed. Such compounds are of the formula: ##STR1## wherein one or more of R.sub.1 -R.sub.8 are various substituting groups. The above compounds react, in the presence of cyanide ion, with compounds containing primary amino groups to form adducts which exhibit a high fluorescent yield and thus are readily detected and measured by fluorometric assaying techniques.