Apple censors references to Chinese politicians, dissidents and other topics in its engraving service, a report alleges.
Citizen Lab said it had investigated filters set up for customers who wanted something engraved on a new iPhone, iPad or other Apple device.
And Apple had a broad list of censored words, not just in mainland China but also in Hong Kong and Taiwan.
Apple said its systems "ensure local laws and customs are respected".
"As with everything at Apple, the process for engraving is led by our values," chief privacy officer Jane Horvath wrote in a letter provided to CitizenLab in advance of the publication of its report.
And the engraving service tried not to allow trademarked phrases, alongside those that "are vulgar or culturally insensitive, could be construed as inciting violence, or would be considered illegal according to local laws, rules, and regulations".
But CitizenLab accuses Apple of having "thoughtlessly and inconsistently curated keyword lists".