Masks are big white glowey things of some plot significance. There are 26 of them. Technically 27, but that one is acquired in a cut scene and doesn't count.
Chapter 1None.
Chapter 2None.
Chapter 3 - 3 masks.
1: Can't miss, in your path.
2: In the area with the sentry and the shielded gunner mech. It is above the ladder that Trip must lower for you. Once you climb it, jump across to the crumbling handholds. Once on the platform, turn around and jump back to land on the mask platform and collect it. Mind the sentry gun that is shooting at you at the time. ;)
3: Can't miss, in your path.
Chapter 4 - 3 masks.
1: At the beginning start climbing the building. On the first platform, head left to the side of the building facing the ravine. Keep going to find the mask on a platform there.
2: Found as you go through the first building.
3: Appears as part of the story. Grab it to start the last part of the level.
Chapter 5 - 3 masks.
1: On the island past the rusted ship, to the left of the bridge.
2: On a platform on the right hand side of the first tower of the bridge.
3: Monkey grabs this automatically.
Chapter 6 - 2 masks.
1: When you get to the first bridge you must lower, don't. Instead go across it and climb the root up to the ledge with the mask.
2: As you approach the 2nd watchtower, it is on a ledge. Go past the heavy door and climb up. Just hop down where the rope fence is absent to get it as soon as you land.
Chapter 7 - 2 masks.
1: Near the start of the level you will see a couch & umbrella facing the cliffside. Turn around and there will be a pole. Climb up it and go across to a platform. The mask is in front of a small house.
2: This is found in the long street with
lots of ammunition. There is an alley to the left near the catwalk with the scout mech that you can take a gun from. Go in that alley, then turn right and the mask is behind the house there.
Congratulations, you now have half the masks!
Chapter 8 - 2 masks.
1: Back near where you found Trip in chapter 7.
2: On the roof of the blue & white structure to the left of the windmill. Climb up it from the windmill after pulling down the first sail. Then hop on one of the poles to the red building. Stop it when the fan blade with the pole in the middle approaches. Get on that and ride it to the other building. Hop off to get the mask.
Chapter 9 - 2 masks.
1: At the start, there is a little path to the left. Follow it to the mask.
2: At the barricade after the turrets.
Chapter 10 - 2 masks.
1: On the walkway underneath the factory logo (look familiar?). When you rejoin Trip & Pigsy, backtrack to get it.
2: To the left of the turret.
Chapter 11 - 2 masks.
1: At the second gun scout attack, on a path to the left, the same raised area that has the health serum. Follow the path to the left to find the mask.
2: In the 2nd area climb up the large centipede mech ruins to find the mask.
Chapter 12 - 2 masks.
1: At the first rail car, don't rotate it. Ride it back to the wall to find the mask. You can't see it because of a render distance limit.
2: When the sub is lowering into the water, there is an open door opposite it. Go through it and hop over some stuff to find the mask.
Chapter 13 - 2 masks.
1: At the first anchor point.
2: In the next room climb up to the fan. You will see it to the left, below.
Chapter 14 - The last mask.
1: After going through the passageway to the other side of the mech, turn around and it is on a platform below.
Below is a video for those of you that like that sort of thing.