Subject: half-life of a radioactive substance really a 'constant'?

I remember some (rare) texts concerning the influence of chemical linkage,
temperature, pressure, &c, on radioactive half-life - but it's is only in my
"bio-computer" and I have no sources. 
Standard texts say, there is no influence, so I'm really sure, there is some
small and usually neglegiable connection. In case you have more links I would
appreciate to get them.
Many thanks in advance!
I beg your pardon in case I made some mistakes, it's my first trial on your site...

Re: half-life of a radioactive substance really a 'constant'?