End of May or end of Aug./early Sept.

I would also chose the late Aug./early Sept. time period.

We stayed in Lech am Arlberg in late August and the weather was perfect. Beautiful green meadows and mountainsides. Hated to leave and go into the heat of Italy!

You could get lucky in late May, but August and September are far more reliable.

Paul,

I'd also suggest end of August and early September (that's the time-of-year I usually travel). I've encountered a few gray and overcast days in that part of Europe, but the weather has generally been good.

Happy travels!

I say go in May before it gets too hot and before the airfare prices go up to Summer levels. The Grossglockner High Alpine Highway over the mountains is open by then.

When I'm planning a trip, I want it to come sooner, not later.

If your visit to the Alps include high-mountain activities (mountain road pass driving, gondolas/cable car rides), beware early June is a transitional period between winter and summer modes. Depending on weather earlier that year, you could have many mountains still thawing (and thus closed) or have conditions for opening already in place. Check schedules of places you might want to visit. Late September is a good time to visit high mountains on summer mode.

Slovenia is better visited on June (not that September is terrible either).

If you want a snow-free trip, go in late August.

Thanks for all the input.

I was leaning towards Aug./Sept. due to some reasons mentioned... cable cars running, passes more accessible, etc. Although I really hate traveling during the warmer/hot periods, this will probably work best. Hopefully being at higher elevations will be a bit cooler, especially in the evenings. We found quite the opposite recently in the Dachstein area though. Early Dec., nearly a foot of snow on the ground, below freezing temp. and during the morning and day when the sun was out, wearing just a sweater. The sun was strong!

If you're going into the mountains, better to go at the end of summer. The trails will be open and the weather is a little more reliable than before summer.

You could have lots of rain in May. I would look for end of August beginning of September.

A budget perspective: Trans-Atlantic air fares tend to go up substantially in mid- to late May and start coming down in mid-September.

We've been to the Alps several times in early September and it's always been very comfortable. You are right that the elevation will protect you from the heat. Have fun!