When it comes to expressing the intensity of one's seriousness, there are various synonyms that can come in handy. Some alternatives for the phrase "was serious about" include words like committed, devoted, earnest, sincere, resolute, determined, dedicated, and fervent. These synonyms are all excellent ways to express the level of importance or significance attached to a particular matter. For instance, one could say "She was committed to her career" or "He was fervent about his beliefs." Using powerful synonyms like these can help to convey the strength of one's convictions and seriousness of intent, adding an extra layer of depth and meaning to communication.