How to tell them apart
たとたん marks an immediate, often surprising reaction triggered the instant an action finishes, like ドアを開けた途端に猫が飛び出した. かと思うと frames two quick successive events the speaker observes, often with a sense of no sooner...than, like 雨が降ったかと思うとすぐやんだ. Swapping them is unnatural because たとたん needs one action directly causing an instant change, while かと思うと links two noticed events without that tight cause-effect bond.
- 途端（に） Use 途端（に） when one action instantly triggers a reaction or change the moment it happens.
- かと思うと Use かと思うと when you notice one event and then another follows right after it.
| Grammar point | Meaning | JLPT level |
|---|---|---|
| 途端（に） ni | the very instant something happens; right after doing something | N2 |
| かと思うと kato uto | the moment after; no sooner than; when realising | N2 |
途端（に） ni
the very instant something happens; right after doing something
途端（に）expresses that something happens the instant another action occurs, often with a surprising or immediate result. It attaches to the past tense (plain past form) of a verb, and に is optional; the pattern is common in both casual and neutral speech.
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暗いトンネルに入った途端、私たちは何も見えなかった。
On entering a dark tunnel, we could see nothing.
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彼女がその本を読み始めたと思った途端、誰かがドアをノックした。
She had hardly begun to read the book before someone knocked at the door.
かと思うと kato uto
the moment after; no sooner than; when realising
かと思うと follows a plain-form verb or a noun/phrase and shows that one thing happens immediately after another, or that a realisation strikes the speaker. It often carries a sense of surprise at how quickly the second event follows the first.
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クリスがどんなにむかつくかと思うと、頭が痛くなりそう。
My head starts to hurt when I think about how annoying Chris is.
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１つが通り過ぎたかと思うと、すぐに次の台風が接近する。
No sooner has one typhoon passed than the next one approaches.
途端（に） and かと思うと are both JLPT N2 grammar points that Japanese learners often mix up. This page compares them side by side on meanings, usage and example sentences, so you can tell them apart.