Are Amazon MP3 downloads in VBR?

it depends entirely on the album

some are CBR, some are ABR, some are VBR

the only constant is that they were all encoded with LAME

FWIW, all the songs I have from Amazon MP3 (which, admittedly are just one album and a few other songs) are VBR V0.

I would think necropimp is right, though, if he's experienced or heard about the variation.

I've only downloaded a couple of albums from Amazon, but they've been in VBR.

you are correct there is no way of knowing unless someone has reported on the encoding

a few known amazon mp3 encodes (just what i can remember off the top of my head)

all Wolves in the Throne Room albums are ABR ~240ishKbps

Bloodbath - Breeding Death EP -V0

Foghat - Fool For The City 256Kbps CBR

Diablo Swing Orchestra - Buthers Ballroom -V0

i'll go through later and report on some others

I suppose if you're concerned enough, you could contact Amazon about the album in question or ask about their encoding practices in general.

I've gotten two albums and they've been V0.

Here is what their Help says about the media downloads

Digital Music Format

Amazon.com's digital music downloads are available in MP3 format—the most widely compatible music file format supported by most media player applications, hand-held music devices, and some CD and DVD players. Here's more information about music files from the Amazon MP3 Music Downloads store:

• Bit Rate: Where possible, we encode our MP3 files using variable bit rates for maximum audio quality and smaller file sizes, aiming at an average of 256 kilobits per second (kbps). Using a variable bit rate allows us to allocate a higher bit rate to the more complex sections of music files while using a smaller bit rate for the less complex sections. The average of these rates is then calculated to produce an average bit rate for the entire file that represents the overall sound quality. Some of our content is encoded using a constant bit rate of 256 kbps. This content will have the same excellent audio quality at a slightly larger file size.
• File Size: A typical 3-minute song takes up approximately 5MB of storage space.
• Download Times: A single MP3 file 5MB in size will download in less than one minute on a typical broad-band Internet connection. A similar download on a 56k dial-up connection can take up to 30 minutes.
• Album Cover Art: Album cover art is included with each song you download.

http://www.amazon.com/gp/help/customer/display.html/ref=hp_mp3land_recformat?ie=UTF8&nodeId=200389400

I think it depends on when they did the encoding.  Their standard a couple of years ago was CBR 256K, and I think they've left a lot of their catalog at that bitrate instead of going back and reencoding.  Anything added to the catalog in the last year or so is VBR-V0, with an average of 256K and a peak of 320K.