Compositionality is the thesis that the meanings of complex expressions are derived from the meanings of their constituent parts and syntactic structure. Questions arise on the relative impact of lexical semantics vs syntax on composition and whether a single vs multiple composition operations are applied. Here, we use a new experimental paradigm to study how syntactic mode of composition (predication vs modification) and lexical semantic properties of the words combined (different adjective classes) influence EEG signals. We compared nouns in sentences composing with a verb (predication) or an adjective (modification) against conditions in which the same noun and adjective, or verb, were separated by a syntactic boundary, preventing composition locally. ERPs and oscillatory responses were modulated primarily by the lexical semantic properties of the words combined, and not by the syntactic mode of composition. N400 amplitudes were larger for nouns after verbs and privative adjectives (e.g., ‘fake’), relative to intersective or subsective adjectives. Alpha and beta band responses were also more prominent after verbs, followed by privative adjectives, subsective adjectives, and intersective adjectives. Our results challenge the neurobiological relevance of linguistic theories that distinguish instances of composition according to syntactic mode of combination, while abstracting away from the specific semantic contributions of the content words involved.