Fatal error: Class 'DOMDocument' not found in /xxx/sites/all/modules/amazontools/amazon.php5.inc on line 328

Can anyone explain why this is occurring?
we have clean update php, and drupal (5.12) versions running

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System Status:

Drupal 5.12
Configuration file Protected
Cron maintenance tasks Last run 2 min 31 sec ago
You can run cron manually.
Database schema Up to date
File system Writable (public download method)
GD library bundled (2.0.28 compatible)
MySQL database 5.0.27
PHP 5.1.2
PHP register globals Disabled
Unicode library PHP Mbstring Extension
Web server Apache/2.2.0 (Fedora)