Heading 2912 covers aldehydes and their cyclic polymers in unmodified form. Once an aldehyde compound has been halogenated, sulfonated, nitrated, or nitrosated, the derivative moves to heading 2913. The presence of one of those substituent groups — confirmed by the chemical specification or CAS number — is the deciding factor.

Because all qualifying derivatives fall into subheading 291300, the main classification question is whether the compound belongs here at all versus in a more specific heading. For example, a nitrated aldehyde that also qualifies as a nitro compound under another heading may need a GRI analysis to determine which heading takes precedence. A binding ruling from CBP can resolve persistent ambiguity.