What is the __utmx cookie?
__utmx is a cookie set by Google Optimize scripts, Google's tool for A/B testing, multivariate testing, and content personalization, typically deployed alongside Google Analytics (Universal Analytics). It appears on a site when the owner has installed a Google Optimize container linked to their Analytics account, usually to run conversion experiments or page variant tests.
Google officially shut down Google Optimize and Optimize 360 in September 2023, so the __utmx cookie is now mostly found on older, unmaintained implementations or sites that still carry leftover scripts that were never migrated to another testing platform.
What data it stores
__utmx stores information about a visitor's inclusion in a Google Optimize experiment and the test variant assigned to them, so the same variant is shown consistently across subsequent visits. The value is an alphanumeric string generated by Google, containing no directly identifying data such as a name or email address.
The data is sent to Google's servers and can be correlated with other Google Analytics identifiers on the same domain (such as __utma or __utmb) to measure the performance of each tested variant.
What it is used for
From Google's side, the cookie ensures experiment consistency: each visitor sees the same page variant across sessions, which is what makes an A/B test statistically valid.
From the website owner's side, __utmx allows comparing the performance of different design, copy, or layout variants and making data-driven conversion decisions, without disrupting the visitor's experience by switching variants on every visit.
Does it require consent?
- Category: statistics / analytics (non-essential cookie).
- Requires prior consent under the GDPR and the ePrivacy Directive (2002/58/EC), as it is not strictly necessary for the website to function.
- Site owners must disclose it in their cookie policy, obtain explicit consent before it is set, and make refusal as easy as acceptance.
- If Google Optimize is no longer actively used, the leftover script should be removed entirely, not just blocked.
How to block or delete __utmx
A visitor can delete or block __utmx manually through their browser's privacy/cookie settings (Chrome, Firefox, Edge, Safari), including clearing browsing data for that domain or enabling third-party cookie blocking.
For site owners, the recommended approach is to prevent the Google Optimize script from running before consent is given. A consent management platform such as CookieFix can automatically block the script until the visitor accepts the “statistics” category, preventing __utmx from being set prematurely.
Frequently asked questions
Google Optimize was officially discontinued in September 2023, so this cookie shouldn't appear on new sites anymore. It can still be found on older implementations with outdated scripts.
It is a persistent cookie with a typical lifespan of about 18 months, stored on the device until it expires or is manually deleted.
It contains an automatically generated identifier with no name, email, or other directly identifying data, but it can be correlated with other tracking identifiers to profile browsing behavior.
Yes, as a statistics/testing cookie it requires prior consent under the GDPR and applicable ePrivacy rules.