There are numerous synonyms for the phrase "immediately upon". A few options include "right after," "as soon as," "directly following," "instantly," "promptly," "forthwith," "without delay," and "in no time". Each of these alternatives offers a slightly different nuance to the timing at which a specific event occurred. For instance, "as soon as" suggests a slightly more immediate action, while "directly following" indicates a sequential order of events. When writing, it's important to vary your word choices to prevent repetition and keep the reader engaged. Alternating between synonyms for "immediately upon" can help improve the flow and pacing of your writing.