We've been evangelising about the power of captions and subtitles for SEO for a long time now. A closed caption file is a form of text file, after all, and it stands to reason that storing what is effectively a text transcript of your video on your site is likely to make the video content more accessible to search engines. We've seen many of our clients' videos rise up the search listings substantially after being captioned, so we've been convinced of the theory, and our clients have been more than happy.

But it's been hard to turn up concrete evidence to prove that captions and subtitles affect SEO... until now. In the last few months it's become clear that YouTube's search results are visibly affected by the caption and subtitle tracks on certain videos, and if you don't believe us take a peek at this image.

This is a screen grab of what happens when we search YouTube for "internetsubtitling". We picked that term because it occurs a lot in closed caption files on YouTube - we include a discreet credit on most of the .srt files we create - but it isn't likely to crop up too often anywhere else. You're unlikely to see it in the titles, tags or metadata of any videos apart from our own.

So, what's so earthshaking about this, exactly? Well, you can see that a lot of DECC's videos have come up as search results. Fair enough - we've captioned plenty of them. But the important thing here is that "internetsubtitling" is not mentioned in the videos or their associated data - there's no way those videos could appear in this search unless YouTube was indexing the caption streams. And the icing on the cake is that you can even click through to the exact place in the video where the search term appears. For YouTube to know where that search term occurs, it must be reading the timecode data in the caption files.

At the moment it's only YouTube's internal search that behaves this way ("only" being a relative term when YouTube fields over 4 billion searches every month). But of course YouTube is owned by Google and runs on Google's search technology, so we'd say it's surely it's only a matter of time before this explicitly caption-assisted search functionality appears in Google's own search results too. Our experience would suggest that captions have been implicitly affecting search results for some time already.

What it boils down to is that captioning or subtitling your YouTube videos now buys them entry to lots of search results lists they'd never normally appear in. And that means captions and subtitles have suddenly become a tremendously powerful SEO tool for driving traffic to your videos, and from there to your website.