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MOBI is the ebook format Kindle was built on. If you have .mobi files sitting in a folder, they are from that era — and the practical situation changed in 2022, when Amazon stopped accepting the format.
MOBI began as the Mobipocket format in 2000, built on an even older Palm document structure. Amazon bought Mobipocket in 2005 and made it the basis of the Kindle.
Underneath, a MOBI file contains HTML — the same basic idea as an EPUB. The two formats are relatives, which is why conversion between them generally works well. What differs is how much of HTML and CSS each supports, and that difference is why MOBI eventually lost.
Several file types belong to this story and are easy to confuse:
The progression tells the story plainly: each generation existed because the previous one could not express enough of what a book needs.
Amazon stopped accepting MOBI for publishing, and stopped accepting it through Send to Kindle — the feature that emails a document to your device.
Two consequences follow, and they are often confused with each other:
So MOBI is not dead on devices; it is closed to new arrivals.
EPUB — the format MOBI competed against for two decades. Amazon now accepts EPUB through Send to Kindle and converts it internally to its own format on arrival.
This is worth stating precisely, because it causes a recurring misunderstanding: the Kindle does not read EPUB. It accepts EPUB, converts it, and stores something else. The file on the device is a Kindle format, which is why you cannot copy it back off and open it elsewhere.
In practice the distinction rarely matters for reading. It matters a great deal if you are thinking of a Kindle library as a collection of files you own in a portable format. It is not.
If it is DRM-free — your own writing, a book from a publisher who sells unlocked files, something from a public-domain library — convert it to EPUB. Conversion is straightforward because both formats are HTML underneath, and EPUB opens on every reading app, every platform, and through Send to Kindle.
Check the result before discarding the original. Conversion handles text and structure reliably; the table of contents, image placement and any unusual formatting are where errors show up.
If it came from Amazon with DRM, it is encrypted and tied to your account. Conversion tools will refuse it, and the honest position is that the book is licensed to you within that ecosystem rather than owned as a file. It will keep working on devices registered to your account; it will not become a portable file.
That constraint is worth knowing in advance rather than discovering later. If the ability to keep and move your own copies matters to you, buying DRM-free EPUB from publishers who offer it is the choice that preserves it.
Not because of corporate politics, but because of what the format could express.
AZW3 fixed much of this, and KFX more, but by then EPUB had become the open standard that everyone else implemented. Keeping a proprietary format that only one company’s devices could read stopped being worth the cost of maintaining it.