Special K will be the exclusive sponsor of iVillage UK's eight-week 'Shape Up Challenge' campaign. The portal expects 40,000 subscribers to sign up.
Special K will run exclusive branding on the challenge's web pages and on an email to 60,000 subscribers. It will also sponsor weekly newsletters sent to 320,000 users.
The brand's wider 'A matter of taste' promotion includes a five-figure spend on specialk.co.uk.
The site was created by inter-active agency magneticNorth, which will also handle an email campaign that will be sent to users on the Kellogg database.
Specialk.co.uk will let users 'taste' different experiences online, such as football, salsa and Berlin, the city in Germany. A competition will offer users the chance to win a dream holiday to a destination of their choice. The site will be supported by press, TV and on-pack activity.
Lou Cordwell, client services director at magneticNorth, said: "Usage and penetration levels are at the right stage to introduce a branded site, which fits well with the promotion."