Zoe and Komnenos mosaics at the south gallery of Hagia Sophia in Istanbul are among the most important examples of Byzantine monumental paintings. We focus on their meaning and subject using a new method. Scholars have generally agreed that both mosaics depict the donations made by Byzantine emperors to Hagia Sophia. The study first analyses their pictorial compositions in a comparative order and aims to deepen the interpretation with historical and iconographical details. The analysis shows us a distinction between two compositions and also indicates a potential difference in subject or meaning. We furthermore argue that historical and iconographical details support this interpretation. In conclusion, it is clear that Zoe mosaic was executed by Emperor Romanos III, between 1028 and 1034, to commemorate his establishment of an annual imperial grant to Hagia Sophia. However, Komnenos mosaic must have been connected with the conflict between Emperor John II Komnenos and his brother Isaakios occurring around 1122. John II declared his son Aleksios co-emperor in 1122, possibly to take precaution against Isaakios. Consequently it seems that John II ordered the mosaic which depicts himself with Aleksios to put his reign and successor forward.