What is the __utmxx cookie?
__utmxx is an HTTP cookie linked to Google Optimize, a Google tool (historically integrated with Google Analytics) used by site owners for A/B testing, multivariate testing, and page personalization. It is set in the browser when a site runs an active Google Optimize experiment.
The cookie appears on the first page load that includes the Optimize script, regardless of whether the visitor interacts with the tested content.
What data it stores
The cookie stores information about the active experiment a visitor is exposed to, such as a test identifier and the variant shown, so the experience stays consistent during the visit and on return. The exact value format can vary depending on how the experiment is configured.
This data is sent to Google's servers, typically alongside other analytics information collected by Google Analytics/Optimize scripts running on the same site.
What it is used for
For the provider (Google), the cookie allows grouping visitors by test variant and measuring experiment outcomes. For the site owner, the practical purpose is comparing multiple versions of a page (design, copy, layout) and finding which performs better, without showing the same visitor different variants on repeat visits.
Does it require consent?
- CookieFix category: statistics (analytics)
- Requires prior consent under GDPR and the ePrivacy Directive (2002/58/EC), since it is not strictly necessary for the site to function
- Site owners must list it accurately in their cookie policy and block the Optimize script until consent is given
- Note: Google shut down Google Optimize in 2023; if this cookie still appears on a site, check whether it comes from a legacy integration or a similar tool
How to block or delete __utmxx
Visitors can delete or block this cookie from their browser settings (Chrome, Firefox, Edge, Safari) under the cookies/site data section, or use private/incognito browsing.
For site owners, the correct approach is a CMP that automatically blocks the Google Optimize script until the visitor consents to the statistics category — CookieFix, for example, does this automatically without manual work on every page.
Frequently asked questions
It does not store directly identifying data like a name or email, but it is classified as an analytics/testing cookie and falls under GDPR because it can contribute to profiling browsing behavior.
The typical reported duration is 18 months, though some implementations may use different durations depending on the experiment setup.
Google officially shut down Google Optimize in September 2023, so this cookie is unlikely to appear on a newly built site, though it may still show up on sites with outdated integrations.
Yes, any statistics or testing cookie set on a site must be explicitly disclosed in the cookie policy, whether it comes from an active service or a discontinued one.