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Data were collected from a brain-computer interface speller that utilized the P3b as a control signal. Stimuli consisted of letters and their “segments”. Importantly, different letters were made up of different numbers of segments from a 10 segment library. Subjects were instructed to mentally note whenever segments from their letter (targets) were flashed. We found that P3b amplitudes of target segments decreased as the number of segments in a letter (target letter complexity) increased.In contrast, the P3b attenuation was not affected by the total number of letters a segment belonged to (segment frequency).These results may reflect higher task difficulty caused by increased working memory load with increased target letter complexity. Alternatively, it’s possible that despite the target rate being fixed at 30% within each block, subjects erroneously believed the target rate increased with target letter complexity.Further work to disentangle these possibilities may enrich our understanding of the P3b.